1 Unconventional Oil and Gas Spills: Risks, Mitigation Priorities, and2 State Reporting Requirements3 Lauren A. Patterson,*,† Katherine E. Konschnik,‡ Hannah Wiseman,§ Joseph Fargione,∥

4 Kelly O. Maloney,⊥ Joseph Kiesecker,# Jean-Philippe Nicot,∇ Sharon Baruch-Mordo,# Sally Entrekin,○

5 Anne Trainor,◆ and James E. Saiers¶

6†Nicholas Institute for Environmental Policy Solutions, Duke University, 2111 Campus Drive, Durham North Carolina 27708, United

7 States,

8‡Environmental Policy Initiative, Harvard Law School, #4123 Wasserstein Hall, Cambridge, Massachusetts 02138, United States

9§Florida State University College of Law, 424 W. Jefferson Street, Tallahassee, Florida 32306, United States

10∥The Nature Conservancy, 1101 West River Parkway, Suite 200, Minneapolis, Minnesota 55415, United States

11⊥U.S. Geological Survey, Leetown Science Center, Kearnevsville, West Virginia 25430, United States

12#The Nature Conservancy, Global Lands Team, 117 E. Mountain Avenue, Suite 201, Fort Collins, Colorado 80524, United States

13∇Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Building 130,

14 Austin, Texas 78758, United States

15○Department of Biology, University of Central Arkansas, 201 Donaghey Avenue, Conway, Arkansas 72035, United States

16◆The Nature Conservancy, African Program, University of Cincinnati, Department of Biological Sciences, 820G Rieveschl Hall,

17 Cincinnati, Ohio 45221, United States

18¶School of Forestry and Environmental Studies, Yale University, 195 Prospect St., New Haven, Connecticut 06511, United States

19 *S Supporting Information

20 ABSTRACT: Rapid growth in unconventional oil and gas (UOG)21 has produced jobs, revenue, and energy, but also concerns over spills22 and environmental risks. We assessed spill data from 2005 to 2014 at23 31 481 UOG wells in Colorado, New Mexico, North Dakota, and24 Pennsylvania. We found 2−16% of wells reported a spill each year.25 Median spill volumes ranged from 0.5 m3 in Pennsylvania to 4.9 m3 in26 New Mexico; the largest spills exceeded 100 m3. Seventy-five to 94%27 of spills occurred within the first three years of well life when wells28 were drilled, completed, and had their largest production volumes.29 Across all four states, 50% of spills were related to storage and moving30 fluids via flowlines. Reporting rates varied by state, affecting spill rates31 and requiring extensive time and effort getting data into a usable32 format. Enhanced and standardized regulatory requirements for33 reporting spills could improve the accuracy and speed of analyses to identify and prevent spill risks and mitigate potential34 environmental damage. Transparency for data sharing and analysis will be increasingly important as UOG development expands.35 We designed an interactive spills data visualization tool (http://snappartnership.net/groups/hydraulic-fracturing/webapp/spills.36 html) to illustrate the value of having standardized, public data.

37 ■ INTRODUCTION

38 Global demand for energy, directives to reduce carbon dioxide39 emissions, and technological advancements in horizontal40 drilling and hydraulic fracturing, have spurred a rapid increase41 in alternative and unconventional energy production over the42 past decade.1 Despite a recent slowdown of activity, energy43 development will likely continue; with shale gas and tight oil44 play production estimated to grow 60% by 2040 compared to45 2015 production.2 Unconventional oil and gas (UOG)46 produced from reservoirs with low porosity and permeability,47 particularly shale reserves, will be a key component of future

48energy development.3,4 Notwithstanding the incredible poten-

49tial for energy development, UOG extraction activities have

50raised concerns about their potential environmental impacts,

51including dewatering streams as well as surface and ground-

52water pollution.5−7 Determination of the ecological impacts of53UOG development is considered a top science priority to

Received: November 14, 2016Revised: February 3, 2017Accepted: February 13, 2017

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54 inform energy policy and conservation and management of55 natural systems.8

56 Hydraulic fracturing enables resource extraction from both57 conventional and UOG wells by injecting high-pressure fluids58 mixed with chemical additives to fracture nonporous, low-59 permeability rock and release the trapped gas or oil.9,10 One of60 the public’s greatest concerns is the impact of surface spills−61 particularly of the chemicals used in hydraulic fracturing−to the62 environment.11

63 Federal law establishes common minimum reporting require-64 ments for some but not all spills (Supporting Information (SI),65 Section A). The Clean Water Act and Oil Pollution Act require66 reporting of discharges that create a sheen on navigable waters.67 The Comprehensive Environmental Response, Compensation,68 and Liability Act (CERCLA) and the Resource Conservation69 Recovery Act (RCRA) require reporting of spills of hazardous70 substances above a threshold volume, but exempt fracturing71 fluids and flowback from this requirement. CERCLA also72 excludes oil spills from reporting requirements. Beyond these73 minimum federal standards, in most cases states determine74 when and how oil and gas related spills are reported75 (exceptions are for wells on federal or Tribal lands, or on the76 Outer Continental Shelf). States often use “spill” and “release”77 interchangeably; hereafter, we generically use “spill”.12,13 State78 law specifies the type of spill event that triggers a reporting79 requirement usually based on whether the spill exceeded a80 threshold volume or concentration and whether it was81 contained or migrated beyond the well site. These rules also82 indicate the reporting method (i.e., verbal or written), timing,83 and content. These factors, as well as the frequency of84 inspection by regulators for independent detection of spills,

85affect which spills are reported and what information are86provided.87Spill data collected by states provide a rich opportunity for88gaining insight into when spills are most likely to occur, where89they are most likely to occur, and the underlying causes. Better90insight into these three factors would provide regulatory91agencies and industry decision-makers with important92information on where to target efforts for locating and93preventing future spills. However, due to different reporting94requirements, the types of spills reported and details about the95type, volume, location, timing and cause of the spills reported96vary across states. Moreover, some states collect or store data in97hard copy or image pdfs, inhibiting analysis unless personnel98later digitize the data. As UOG expands, efforts to reduce spill99risk would benefit from making data more uniform, accessible,100and informative to well operators, regulators, and the public.101Objectives. In this paper we cleaned and analyzed state spill102databases related to unconventional oil and gas (UOG) wells in103four states: Colorado,14 New Mexico,15 North Dakota,16 and104 f1Pennsylvania17 (Figure 1). We defined UOG wells as those105having been hydraulically fractured, as explicitly identified in106the databases we acquired, or, where this information was107unavailable, as indicated by horizontal drilling or water use.108These states were selected because of their significance as oil or109natural gas producing states and because extensive spill data110could be obtained. We assessed data availability for 11 other111states; however the data were either difficult to obtain or we112could not determine which wells were linked to UOG activity,113leaving four states in this study. We focused on UOG wells as a114manageable subset of oil and gas related spills and one of115growing importance as UOG activity increases. Although some116of the spills that occur at UOG wells are unique to UOG

Figure 1. Location of UOG wells and reported spills.

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117 development, such as spills of fracturing chemicals and118 flowback, other spills, such as spills of produced water, can119 occur at both conventional and unconventional wells.120 Our objective was two-fold. First, we assessed the regulatory121 framework for how spills were reported in each state and how122 that shaped the data collected. Second, we quantified (1) the123 probability of a spill by well age (with spud−initial drilling−124 year representing age 0), (2) the risk of a spill occurring at125 different locations on a well pad, (3) the volume released, and126 (4) the underlying cause of the spill. The regulatory assessment127 and the spill data analysis were used to ascertain how improved128 data collection and standardization procedures might enable129 higher resolution data analysis that could decrease the130 likelihood for spills. We built an interactive database to131 showcase this opportunity (http://snappartnership.net/132 groups/hydraulic-fracturing/webapp/spills.html).

133 ■ MATERIALS AND METHODS

134 We obtained state oil and gas spills data for spills occurring135 between 2005 and 2014 at UOG wells that were spudded as136 early as 1995. We constrained UOG well spud dates to limit the137 range of technology and techniques to unconventional shale138 well activity18 and capture the transition to mainstream UOG139 activity. Analysis of spills data was limited by difficulties140 obtaining earlier data and little UOG production related to141 shale gas or tight oil plays occurred prior to 2005.2 Spill data142 were matched by American Petroleum Institute (API) numbers143 to UOG wells. Data for Colorado UOG wells were obtained144 from the IHS Enerdeq database,19 and from state databases in145 New Mexico,20 North Dakota,21 and Pennsylvania.22 When the146 distinctions between conventional and UOG wells were not147 clear, we did not include the well to avoid including non-UOG148 wells in our data set. Pennsylvania was the only state to provide149 a separate well database for unconventional wells. The IHS data150 for Colorado provided the volume used to complete wells so151 we included all horizontal wells and those wells using more152 than 1 Mgal for completion. In North Dakota and New Mexico,153 only horizontal wells were included. Further detail on how we154 identified UOG wells in each state can be found in Section B of155 the Supporting Information.156 Spill Data. Spills are events in which an unauthorized157 discharge released material, regardless of whether the spill158 stayed within the boundaries of the pad or migrated into159 groundwater or surface waterways. For a spill to occur there160 must be (1) a means by which materials were released from a161 particular point at or near the well site (hereafter referred to as162 a pathway) and (2) a causal mechanism (such as human error163 or equipment failure) that resulted in a release from the164 particular pathway. For example, a tank (pathway) may release165 produced water to the surface due to equipment failure (causal166 mechanism) from a valve malfunction. We focused on167 identifying the pathway and the causal mechanism for each168 spill to determine the most likely routes for spills and thereby169 suggest where monitoring, development of new practices, and170 focused interventions may have the most benefit. A detailed171 analysis of the materials spilled and potential environmental172 impacts of spills is described in a separate paper.23

173 The best source of information regarding UOG spills are174 state reporting records. Each state’s reporting thresholds and175 requirements differ, such that the subset of spills that are176 reported vary by state (SI, Section B provides details on state177 spills data). The number of spills reported is likely conservative

178given that only a subset of all spills are required to be reported179to each state.180Colorado. In 2000, an interagency memorandum required181companies to verbally report any spill entering or threatening to182impact surface waters as soon as practicable.24 In addition,183companies were required to report spills exceeding 5 barrels184(210 gal) on state Form 19 within 10 days of discovery (SI185Figure S2). In 2014, the reporting thresholds were lowered and186time periods for reporting were shortened;25 requiring any spill187or release greater than 1 barrel (42 gal) that escapes secondary188containment to be reported. Also in 2014, Colorado amended189Form 19 to capture information about the material spilled and190the cause for the spill.25 Spills data were obtained through the191Oil & Gas Conservation Commission.192New Mexico. New Mexico’s Oil and Conservation Division’s1932001 rules required companies to verbally report within 24 h194any “major release” of oil, gas, produced water, condensate, oil195field waste including regulated radioactive materials or other oil196field related chemicals, contaminants, or mixtures.26 A major197release was defined as greater than 25 barrels (1050 gal), or a198release that resulted in a fire or in substantial damage to199property or the environment, would reach a water course, or200might “with reasonable probability endanger public health” or201water quality.27 In addition to the verbal reports for major202releases, written reports were required within 15 days on Form203C-141 to document any “major release” or “minor release”, that204is, releases of at least 5 barrels (210 gal). The location and cause205of the spill must be reported, along with the material and the206source of the release, as well as whether the release reached a207waterway. The Division reorganized the rules in 2008, but the208substantive reporting provisions remained the same.27 Spills209data were obtained from the Oil Conservation Division.210North Dakota. Prior to 2010, companies in North Dakota211had to report “any fire, leak, spill, blowout, or release of fluid”212except for spills less than 1 barrel (42 gal) that stayed on site.28

213Reports, which were strictly verbal, had to include the location214and cause of the incident, and the amount and type of fluid215involved. Effective April 1, 2010, a time limit was placed on this216verbal report (within 24 h of discovery); in addition, a written217report was required to be submitted within 10 days of cleanup218“unless deemed unnecessary by the director” of oil and gas of219the Industrial Commission.29 Effective April 1, 2014, North220Dakota added an online reporting requirement within 24 h of221discovery.30 Spills data were obtained from the Oilfield222Environmental Incident reports provided by the Department223of Health-Environmental Health. These data do not include224RCRA spills.225Pennsylvania. Pennsylvania’s 2001 rules required companies226to report by telephone to the Department of Environmental227Protection any “reportable release of brine” or the discharge of228any substance which would endanger downstream users of229water, result in or create a danger of pollution of Pennsylvania230waters, or damage property.12,31,32 The report had to include231the location and cause of the incident. “Reportable release of232brine” was defined as “spilling, leaking, emitting, discharging,233escaping or disposing” of at least 5 gallons in 24 h of brine234containing more than 10,000 mg/L total dissolved solids235(TDS), or of at least 15 gallons of brine with a lower TDS236concentration. In October 2016, Pennsylvania’s new rules went237into effect; these will require written spill reports.33

238Pennsylvania does not have a separate spill data set, therefore239spill data for our analysis were pulled from the Department of240Environmental Protection’s notice of violations (NOV) data-

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241 base for UOG (SI, Section B). This necessarily limited the spill242 data to those where an inspector issued an NOV, possibly243 leading to an underestimation of the number spills in our244 analysis. Using the violation code and comments, we included245 in our analysis only those NOV’s categorized as having a spill or246 the potential to result in a spill (n = 1293).247 Data Cleaning. To ensure consistency among states, the248 same analyst cleaned each state database. The data provided by249 each state were reconciled with a consistent classification250 system for the volume units, material, pathway, and cause of251 spill. Some states provided the data in separate fields, while252 others only provided the information through narrative253 descriptions. When there were conflicting data for a spill254 (e.g., the cause or volume of the spill differed from the255 narrative), the narrative was taken to be correct. Spill volumes256 were converted from reported units (typically barrels) into257 cubic meters and gallons. The material spilled was categorized258 as drilling waste, chemicals, hydraulic fracture solution,259 saltwater, freshwater, oil products, diesel, equipment oil, and260 unknown. We employed similar methods to those reported in261 an EPA analysis of eight states34 in that we used one analyst262 and treated narrative descriptions as the governing factor to263 identify common pathways and causal mechanisms. However,264 we focused on spills at all stages of well development while the265 EPA analysis focused on spills connected to the hydraulic266 fracturing process.267 Pathway categories were developed based on descriptive268 narratives of incidents. Pathways identify the specific point on269 or near the well site at which a spill occurred, except the270 blowout pathway, which describes an event in which pressure271 builds up in the wellbore and can lead to underground or272 surface releases. Pathways that were relatively small and273 occurred infrequently were lumped into a category of “other”274 (SI, Section C). More commonly reported pathways include:275 blowouts, drilling equipment (active mud system, drill rig, and276 shakers), completion equipment (blender, chemical totes, and277 storage containers), tanks, pits, flowlines, heater treaters,

f2 278 stuffing boxes, pumps, transportation, leaks around the

279 f2wellhead, and unknown (Figure 2; SI Table S3). Across all280states, between 93% (PA) and 98% (ND) of spills fell into281these main pathways.282Of the four states assessed in this study, only Colorado and283New Mexico categorize the underlying cause of a spill in their284aggregated data sets. We folded their standardized designations285into four general categories: equipment failure, environmental286conditions, human error, and unknown. We used narratives and287other reported data from North Dakota and Pennsylvania to288assign a causal category to each spill in those states as well.289Equipment failures ranged from leaks due to corrosion to290specific mechanical problems within equipment such as valves,291flanges, gaskets, sight glasses, fire tubes, and polish rods. Spills292from equipment that failed due to extreme conditions, such as293cracked valves due to freezing, were listed as spills caused by294environmental conditions. Other environmental conditions295included flooding, heavy rain, snowmelt, high winds, rock296slides, well kicks (incidents in which pressure causes brines and297other fluids to enter the wellbore), valves opened by cattle298coming into contact with equipment, and gophers chewing299through flowlines. Human errors included incorrect valve300positions, miscommunication between well operators and301transportation staff, flowlines being punctured by construction302equipment, vandalism, and illegal dumping.303Analysis. We normalized the data four different ways to304compare spill rates between states: the average rate from 2005305to 2014, annual rate, life-year rate, and the combination of306annual and life-year rates. We use the term “rate” to refer to the307occurrence rate (frequency) of spill events. Examples for all308calculations can be found in the SI Calculations spreadsheet.309We calculated the average spill rate by dividing the total310number of spills by the number of well-years observed within311the state. A well-year is observed each year since the well was312drilled and spill data were obtained (2005−2014). For example,313a well drilled in 2010 would have five well-years observed (from3142010 to 2014) for a spill in 2014, but if that well had been315drilled prior to 2005, it would have 10 well-years observed316(from 2005 to 2014).

Figure 2. Common pathways for spills.

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317 We calculated the annual spill rate by dividing the number of318 spills in a given year by the cumulative number of wells drilled,319 including that year (see SI Calculations spreadsheet).320 Given the rapid and recent expansion of unconventional321 wells, most of our observations are for young wells (median322 well age in 2014 was 3 years). It may not be appropriate to323 extrapolate this spill rate over the entire lifetime of a well.324 Therefore, we also calculated the life-year spill rate for each life-325 year of a well (i.e., all new wells, all one year old wells, etc.). We326 based the life-year of a well on its spud date. We rounded spills327 to the closest life-year, such that life-year 1 represents spills that328 occurred between six and 18 months after a spud date. Life-329 years ranged from 0 (wells spudded in 2014) to 19 (wells330 spudded in 1995) (SI Table S7). While spills may have331 occurred at some of these wells prior to 2005, we only have332 spill data from 2005 to 2014.333 The volume of material spilled was assessed using quantile334 regressions (quantreg package in R) at 10% intervals. For each335 state, a chi-square test was performed between the observed336 and expected number of wells with multiple spills. The337 expected number of wells is based on the binomial probability338 of more than one spill occurring at a well.

339 ■ RESULTS AND DISCUSSION340 Average Spill Rates. Between 2005 and 2014 there were341 6648 spills reported across the four states based on each state’s342 reporting requirements and our definition of UOG wells (SI343 Table S4). Our results exceed the number of spills found by344 EPA (n = 457) for eight states between 2006 and 2012 because345 we included spills that occurred during all stages of unconven-346 tional production (from drilling through production) while347 EPA focused on those spills explicitly related to hydraulic348 fracturing.34 In our study, North Dakota reported the most349 spills (n = 4453) and the highest overall spill rate at 12.2% (SI350 Table S6). Pennsylvania reported 1293 spills (4.3%), whereas351 New Mexico (426 spills; 3.1%) and Colorado (476 spills; 1.1%)352 each reported fewer than 500 spills. Some wells within each353 state experienced multiple spills (from 0.12% of wells in354 Colorado to 7.32% of wells in North Dakota); and these wells355 with multiple spills contributed to a large proportion of spills.356 The proportion of spills coming from a well with more than357 one spill accounted for 26% of all spills in Colorado, 37% in358 New Mexico, 53% in North Dakota, and 47% in Pennsylvania.359 The number of wells with multiple spills was 1.3 (North360 Dakota) to 4.6 (Colorado) times higher than expected due to361 chance alone, indicating that wells with one spill have a higher362 probability of spilling in the future (SI Table S5).363 Annual Spill Rates. Within New Mexico and Colorado the364 annual spill rate fluctuated within a percent or two; whereas in365 North Dakota and Pennsylvania, the annual spill rate fluctuated366 by more than 13% (SI Table S6). Fluctuations were likely367 influenced by changes to state spill reporting requirements,368 demonstrating how state policies directly impact efforts to369 identify and accurately assess UOG risk, their causes and370 potential mitigating remedies. For instance, North Dakota’s371 transition from a verbal notification to a written reporting372 requirement coincides with an increase in spill rates of 3−4%

f3 373 after 2010 (Figure 3). Similarly, the lowering of a reporting374 threshold for a spill could increase the number of reported375 spills. From 2000, Colorado only required spills larger than 210376 gallons to be reported; after 2014, the state lowered the377 threshold to 42 gallons when a spill escaped secondary378 containment. This change occurred during our last year of

379analysis, and we did not discern a noticeable change in spill380rates from those few months of data. The requirement that the381spill escape secondary containment may also drive down spill382reports. Colorado also provided unique identifying information383for pits, tanks, flowlines, etc., but it did not always link these384spills to an individual well. This reporting method may have385contributed to lower Colorado spill rates since we only386included spills that could be linked to an unconventional well387API.388Pennsylvania’s reporting requirements remained the same389throughout this time period; however, a spike in UOG well390development in 2009 corresponded with a doubling of the391number of inspectors from 35 to7635 and a spike in annual spill392rates (Figure 3). The rates declined as the number of wells393increased (1208 wells in 2009 to 4478 wells in 2011), due to a394combination of factors including the centralization of regulators395into a new Office of Oil and Gas Management within the396Pennsylvania Department of Environmental Protection, oper-397ators gaining more experience with best management practices,398and a shift in inspection to focus on drilling which increased the399number of wells that could be inspected while simultaneously400decreased the likelihood of finding a violation.36 Frequency of401inspection is a critical factor for reporting spills in Pennsylvania402since spills data were obtained from notices of violations.403Meanwhile, pit spills decreased from 132 in 2010 to 28 in 2011,404which may correspond with an increased effort by the405Pennsylvania Department of Environmental Protection to406enforce pit regulations (S. Perry, Pennsylvania Department of407Environmental Protection, Personal Communication, May 25,4082016). Spill rates increased in New Mexico between 2012 and4092013, corresponding with increased production. Over this410period, storage was inadequate to contain the increased411quantities of produced water and oil, and some truckers412determined it was cheaper to illegally dump produced water413than to find adequate storage (D. Sanches, New Mexico Oil414Conservation Division, Personal Communication, June 1,4152016), which may have accounted for the increased spill rate.416Life-Year Spill Rates. The spill rate by life-year of the well417was greatest in the first few years (SI Table S8) when drilling,418completion, and the highest amount of production oc-419curred.37,38 The average spill rate ranged between 2%420(Colorado) to 15% (North Dakota) during the first three421years of well life; with rates decreasing as the well matured. We422quantified the spill rate by life-year when at least 100 well years423 f4were present to increase the robustness of the analysis (Figure424 f44). Our data set only included new wells spudded from 1995;425further research is needed on spill rates for older wells426exceeding 20 years of age.

Figure 3. Annual spill rate. Dashed vertical lines represent changes inreporting requirements.

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427 Annual Spill Rate by Life-Year. Given the observed428 change in spill rate between younger and older wells, it was429 necessary to control for life-year to understand changes in spill430 frequency during the decade we analyzed. We examined spill431 rate by year and life-year to differentiate whether spill rates432 were influenced by improvements in technology or manage-433 ment practices over time or by the age of the well. For434 Colorado, North Dakota, and Pennsylvania the maximum spill435 rate was consistently within the first three life-years of the well

f5 436 over time (Figure 5). New Mexico experienced an increase in437 spill rate across all life-years in 2013 when production began to438 rapidly increase.

439Spill Volume. Reported volumes of spills ranged from as440small as 0.004 m3 (1 gal) to as large as 3756 m3 (991 200 gal).441Spill volume was not always reported. The frequency of a spill442report including the volume spilled varied between states,443ranging from 27% in Pennsylvania to 98% in North Dakota (SI444Table S9). Pennsylvania may have “missing” volumes data445because reporting of spills has only been required by telephone;446agency guidance discouraged written notification.39 The 2016447regulations will require a written report for spills exceeding 42448gallons or when a spill threatens to pollute Pennsylvania449waters.33,40 Another reason for missing volumes data is that the450spills data for Pennsylvania comes from its enforcement

Figure 4. Spill rates by life-year of well (left) and number of UOG wells that experienced each life-year (right). New wells included those with a lifeyear of 0−2 years (3 total years).

Figure 5. Spill rate by life-year of the well. Bubble size is scaled to the maximum spill rate that occurred in each state. The maximum spill rate is given(4% in CO, 9% in NM, 23% in ND, and 12% in PA).

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451 records. Inspectors may not have known the volume of spills452 encountered after the fact, or may have observed a potential453 spill that had not yet manifested into an actual spill.454 When volumes were reported, the median volume of455 reported spills in Colorado (3 m3 or 798 gal) and New Mexico456 (4.9 m3 or 1302 gal) were larger than North Dakota (0.8 m3 or

f6 457 210 gal) and Pennsylvania (0.5 m3 or 120 gal) (Figure 6). The458 larger median sizes in Colorado and New Mexico may reflect459 the higher volume reporting threshold relative to Pennsylvania460 and North Dakota (SI Figure S2).

461 The total volume of reported spills from 2005 to 2014 was462 970 m3 (0.26 Mgal) in Pennsylvania, 5373 m3 (1.42 Mgal) in463 Colorado, 6562 m3 (1.73 Mgal) in New Mexico, and 33 850 m3

464 (8.94 Mgal) in North Dakota. If we took the median spill465 volume by state and applied it to those spills without a reported466 volume, these estimates increase to 1447 m3 in Pennsylvania,467 5488 m3 in Colorado, 6621 m3 in New Mexico, and 33 937 m3

468 in North Dakota. There were 46 reported freshwater spills, of469 which 41 occurred in North Dakota, including 10 spills that470 exceeded 100 m3 (26 417 gallons). The reported volume spilled471 in PA would fill 40% of an Olympic-sized pool; removing472 freshwater spills, the volume reported in North Dakota would473 fill 8.8 Olympic-sized pools. If we limit our analysis to those474 materials associated with hydraulic fracturing (chemicals,475 hydraulic fracture solution, and flowback), the total volume of476 reported spills ranged from 377.2 m3 (0.1 Mgal) in477 Pennsylvania to 769.3 m3 (0.2 Mgal) in Colorado. A more478 in-depth analysis by material and impacts to surface waters from479 this data set are explored in a separate analysis.23 Unlike spill480 rates, the volume of spills was not clearly related to the life-year481 of the well (SI Figure S3).482 Pathway Analysis. In the majority of their incident reports,483 neither North Dakota nor Pennsylvania provided pathway484 information, with many pathways being discerned from485 narrative descriptions, resulting in 1322 unknown pathways

f7 486 across the two states (Figure 7). For the spills that did have a487 reported pathway, the most common pathways across all states488 were related to storage (tanks = 1405 spills, pits = 322 spills,489 pits or tanks =190 spills) and flowlines (N = 1393 spills);490 accounting for 50% of spills. This finding comports with491 research done in North Dakota that found tank and flowlines492 were major pathways for spills.41 In addition, spills related to493 transporting materials by trucks were prevalent in all four states

494(N = 519 spills), with the majority of those spills related to495issues of loading and unloading material (N = 452; 87%).496North Dakota experienced a large number of spills related to497heater treaters and stuffing boxes (N = 682), while498Pennsylvania had a high number of pit-related spills (N =499345), the majority of which occurred prior to 2011.500The data indicate that for every 1000 wells drilled, 12 wells501experienced a tank related spill and 11 wells experienced a spill502 t1related to flowlines (Table 1). Approximately one well per503thousand had a blowout, which is comparable with reported504unconventional blowout frequencies of 1−3 per 1000 wells in505Texas42 and 1 blowout per 1000 onshore wells drilled between5061975 and 1990.43 In another study, the number of blowouts507reported at California oil and gas wells decreased by 80% to 0.2508blowouts per 1000 wells due to improvements in production509practice.44 Issues related to well casing and well communication510are rare (<1 per 1000 wells) according to our data. However,511underground releases are likely under-represented in the spills512database because it is difficult to ascertain when a subsurface513spill occurred. For example, the Notice of Violations database514in Pennsylvania found that between 2.6% and 6.2%10 of wells515had structural integrity issues, which was much higher than516reported in the spill data of the other three states (Table 1). In517North Dakota and Pennsylvania, the pathway was unknown in51824 and 16 of 1000 spills, respectively. In Pennsylvania,519unknown was the predominant pathway.520Reported spill volumes varied widely for most pathways (SI521Figure S4). The largest spill reported in all four states was a522freshwater tank spill of 3752 m3 (991 200 gal) in North Dakota.523North Dakota also received reports of a saltwater flowline leak524and another freshwater tank leak that each released more than5252688 m3 (710 000 gal). The maximum reported spill in526Colorado was attributed to a blowout with 731 m3 (193 200527gal) released followed by a pit spill of 636 m3 (168 000 gal).528New Mexico’s largest spills were related to the wellhead; for529instance, a spill of 372 m3 (98 280 gal) resulted from well530communication, while a blowout released 364 m3 (96 600 gal).531Pennsylvania had by far the smallest number of spill volumes532reported, making it unclear whether the relatively low533maximum spill volume of 67 m3 (17 640 gal) from a tank is534accurate.

Figure 6. Quantiles of reported spill volumes by state. For instance,half of spill volumes in New Mexico were less than 4.9 m3, while halfwere less than 0.5 m3 in Pennsylvania.

Figure 7. Number of spills reported for each state by pathwaycategory. Spills reported as pits or tanks in Pennsylvania were dividedequally between the two categories.

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535 Causal Mechanism. In Colorado and New Mexico, the536 cause of the spill is explicitly asked for in the reporting form,537 resulting in nearly 90% of incident reports providing an538 underlying cause. In contrast, no causal mechanism could be539 attributed to the majority of spills in North Dakota and

f8 540 Pennsylvania (Figure 8), where such information is not actively

541 solicited. Additional insights related to volumes and rates

542 associated with causal mechanisms for pathways can be found543 in SI (Section E).544 Data Reporting and Policy Implications. The primary

545 regulatory responsibility for UOG development lies with the

546 states.45,46 This includes spill reporting requirements. Our

547 findings suggest that the differences in these reporting regimes

548determine the quantity, quality, and usability of data received by549the state.550Data Collection. For states to accurately profile the risks of551pollutant releases associated with UOG production and tailor552regulations to these risks, spill data need to be collected. States553that use low minimum volume thresholds for reporting554requirements can quantify and potentially address the555cumulative impacts of low volume spills. Standardized data556should be collected within and across states to facilitate future557analyses and guide policy, practice and mitigation. In the states558we reviewed, the reporting thresholds ranged from 5 gal559(Pennsylvania) to 210 gal (New Mexico) (SI Figure S2). A560threshold quantity for reporting is logical so that operators and561regulators are not overwhelmed by reports of de minimiz562quantities of substances being released to the environment. But563if the state spills threshold is too high, valuable information564about the types and frequencies of spills occurring will be565missed. Moreover, a number of smaller incidents at a site or,566associated with a particular company, or from conditions such567as an unusually cold winter and freezing of valves, can indicate568problems that could be addressed before a larger incident569occurs. In addition, more subjective thresholds may not be clear570enough to induce reporting. For instance, a release of less than571210 gallons in New Mexico should be reported if it “may with572reasonable probability be detrimental to water”.26 This573subjective standard is unlikely to lead to consistent reporting.574Finally, limiting reporting to spills outside of secondary575containmentas Colorado doesmay tell a more accurate576story about potential impact, but also reduces the opportunities577to identify and mitigate risks within secondary containment578before they pose a problem to the environment by escaping579containment.

Table 1. Annual Number of Spills Per Thousand Well-Yearsa

pathway colorado new mexico north dakota pennsylvania total

blowout 0.2 0.5 2.0 0.1 0.8

process drilling 0.5 0.2 0.6 0.7 0.5completion 0.3 0.4 0.2 0.6 0.4

storage tanks 3.5 10.1 27.7 6.8 12.3pits 0.5 0.8 1.2 11.5 3.4

flowlines 2.2 6.9 31.4 2.0 11.4

transportation 1.6 1.9 10.3 1.7 4.3

equipment heater treater 0.1 1.6 10.4 0.0 3.3pump 0.5 2.1 3.1 0.2 1.4stuffing box 0.2 0.8 8.2 0.0 2.6

wellhead blowout preventer 0.1 0.2 0.1 0.0 0.1free water knockout 0.0 0.8 0.1 0.0 0.1separator 0.4 1.3 0.6 0.2 0.5well casing 0.1 0.2 0.0 0.6 0.2well communication 0.0 0.1 0.0 0.0 0.0wellhead 0.6 1.0 0.1 0.3 0.5

other 0.3 1.4 2.2 2.7 1.6unknown 0.4 1.1 23.6 15.5 11.1total 11.4 31.4 122.0 43.0 54.6

aThe most frequent pathway is highlighted for each state.

Figure 8. Percent of spills by causal mechanism.

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580 Each state required reporting of the pathway of the spill;581 however, only Colorado and New Mexico provide separate582 fields asking operators to describe the pathway and the cause583 (equipment failure, human error, etc.). Only 3% of pathways in584 both states were unknown; whereas 19% of North Dakota and585 36% of Pennsylvania spill pathways were unknown.586 Finally, the timing for reporting is important. The sooner an587 operator is asked to record a description of a spill, the more588 likely the information will be accurate and based on eye-witness589 accounts. Colorado, New Mexico, and North Dakota have590 immediate reporting deadlines (i.e., within 24 h) for large591 events, but no state requires this initial report to be in writing.592 Follow-up written reports are due within 3 days in Colorado,593 10 days in North Dakota, and 15 days in New Mexico. Before594 October 2016, Pennsylvania’s rules did not set a deadline for595 reporting; the Commonwealth’s new rules will require596 reporting of larger or more harmful spills in writing within 15597 days of discovery.40

598 Data Accessibility. Further, the data need to be accessible so599 that the data can be readily analyzed to identify the most600 relevant risks to the benefit of a variety of stakeholders601 including industry, academia, regulators, and NGO’s. Regard-602 less of the user, if data are not presented in a standardized,603 tabular format, it is difficult to analyze spill data effectively; this604 condition is prevalent across states.34,47 The biggest hurdle to605 using data, as currently collected, is the time and resources606 required to make it usable. Colorado, New Mexico, and North607 Dakota require written reporting; as will Pennsylvania once its608 October 2016 rules go into effect.33 Verbal reporting is not only609 less reliable, but it places the onus on the regulator receiving610 the call to take down the correct information and to upload it611 into a system where it can be integrated with information from612 other calls. Pennsylvania’s verbal reports were not available to613 assess, requiring reliance on Pennsylvania’s notice of violation614 database to identify potential spills.615 Once a state requires written reporting, the next step is to616 craft the reporting form to standardize the collected617 information. This makes it easier to compare information618 across incident reports. Online reporting eases processing and619 shortens the timeline for making data available. Colorado and620 North Dakota use online report submissions, with checkboxes621 and drop-down menus to elicit standardized answers. This622 enables a consistent taxonomy of spills to help regulators and623 companies spot risk patterns more efficiently. The forms still624 elicit a narrative response about the pathway of the spill and625 cause of the spill, which provides greater detail and nuance than626 checkboxes. The pdf forms are completed and submitted627 electronically. In addition North Dakota’s form cannot be628 submitted until key fields are completed, reducing the amount629 of missing information. New Mexico also provides an online630 form, but it must be downloaded and submitted in person or by631 mail to one of four district offices where the data are then632 entered electronically. Moreover, most of New Mexico’s form633 calls for narrative responses to such questions as volume of the634 release and a description of the incident. While narratives are635 rich sources of information, much additional work by the end636 user is required to make the information accessible in New637 Mexico.638 Going forward, additional information might be useful; for639 instance, whether a spill occurred during construction, drilling,640 fracturing and completion, production, and/or during main-641 tenance events. States also might consider adding more options642 in their drop-down menus. Approximately 14% of entries in

643North Dakota’s online form resulted in the type of incident644being labeled as “other”. The narratives accompanying these645spill reports may suggest additional common pathways, which646could be added to the menu.647Now that Pennsylvania’s new rules require a written report648for some spills, the Commonwealth might consider creating an649online form with standardized categories of response and, with650space for narratives to elicit details that cannot be captured651through standardized reporting.652Converting Data to Information. State spill data hold great653promise for risk identification and mitigation. About 15% of all654wells reported a spill, with more than 75% occurring within the655first three years of well life. State spill rates indicate that for656every 1000 wells there were 11 spills in Colorado to 122 spills657in North Dakota. However, this comparison is not apples-to-658apples given the differences in reporting thresholds between the659states, with North Dakota having a lower reporting threshold660than Colorado (SI Figure S2). Additionally, we found between66126 and 53% of spills occurred at wells that experienced more662than one spill, perhaps indicating wells with chronic problems.663The most common pathways were tanks and flowlines,664followed by pits, transportation, and heater treaters. Additional665safety practices, training and failsafe technology could666significantly reduce the prevalence of these spills;48−50 for667example, spills associated with the loading and unloading of668trucks. Based on this information, companies and agencies669might direct resources to improve supervision of the loading or670unloading process and better training of employees involved in671this process. Or, industry might focus on improving672technologies to prevent or mitigate these spills, such as well673pad liners or other secondary containment placed beneath674loading and unloading operations. Colorado recently undertook675an analysis of their spills data and found flowlines to be a major676cause of spills.51 As a result, the agency issued guidance on677flowline installation and maintenance.52 This type of678information could also be used by the oil and gas industry to679improve internal line inspections.680Similarly, the high rate of spills from valves at the wellhead681suggests a need for better securing or protecting the valvesor682simply fencing the wellhead or well siteto prevent wildlife683and livestock from jostling the valves and causing spills. And684spill rates from flowlines cut during excavation and construction685suggest that clearer marking of flowline location and the use of686flowlines with puncture- and corrosion-resistant material could687reduce these incidents. Identification of these types of risks688could also inform leading practices for industry and permit689conditions for regulators. As UOG expands internationally,690focusing monitoring on high risk pathways will provide the best691opportunity to reduce risk from surface spills.692Further improving reporting requirements and processes for693reporting will facilitate states’ and companies’ efforts to identify694risks for certain types of spills and take action to mitigate some695of the identified risk factors.46,53 To the extent that this696information is publicly available and searchable, operators can697use it to remove or mitigate risk factors to improve698environmental performance and avoid higher insurance699premiums.37

700Assembling these data electronically within a centralized701database would allow state regulators and other stakeholders to702identify trends, including the most common spill pathways and703causes, as well as identify the wells or operators associated with704unusually high spill rates. Making this information publicly705available and providing it in an easy, usable format would allow

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706 operators, insurance companies, and citizen monitoring groups707 to assess the largest and most prevalent risks and respond708 accordingly. This paper illustrates the benefits of having709 available and accessible data. We found that most spills occur710 within the first 3 years of well life. We quantified the decrease in711 spills following changes in pit regulation in PA (Figure 3), and712 the increase in spills in New Mexico following rapid increases in713 production (Figure 5). We found most spills to be related to714 storing materials (tanks and pits) and moving materials715 between equipment (flowlines). We demonstrate the value of716 this data by creating an interactive data visualization tool where717 users can filter spills and explore changes in rates, volumes,718 pathways and underlying causes (http://snappartnership.net/719 groups/hydraulic-fracturing/webapp/spills.html).

720 ■ ASSOCIATED CONTENT721 *S Supporting Information722 Data used for this study can be accessed here: https://doi.org/723 10.5063/F1TB14VK. The Supporting Information is available724 free of charge on the ACS Publications website at DOI:725 10.1021/acs.est.6b05749.

726 Materials related to how UOG data were extracted for727 each state, detailed description of pathways, and728 additional tables and figures showing results from this729 analysis (PDF)730 A spreadsheet with all calculations (XLSX)

731 ■ AUTHOR INFORMATION732 Corresponding Author733 *Phone: 919-613-3653; e-mail: lauren.patterson@duke.edu.734 ORCID735 Lauren A. Patterson: 0000-0002-7512-7845736 Notes737 The authors declare no competing financial interest.

738 ■ ACKNOWLEDGMENTS739 We thank the editors, three anonymous reviewers, and USGS740 reviewers for their comments; greatly improving the manu-741 script. This work resulted from the SNAPP: Science for Nature742 and People Partnership Impacts of hydraulic fracturing on743 water quantity and quality Working Group at the National744 Center for Ecological Analysis and Synthesis, a Center funded745 by the Gordon and Betty Moore Foundation, the University of746 California, Santa Barbara, and the State of California. J.P.N. also747 thanks the database provider IHS (http://www.ihs.com) for748 access to the Enerdeq database. Use of trade, product, or firm749 names does not imply endorsement by the U.S. government.

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