Cover photo: Folded Paleozoic strata along east flank of the … · 2018-09-07 · Cover photo:...

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Cover photo: Folded Paleozoic strata along east flank of the Cacapon Mountain anticline, northeastern WV, looking southeast near Berkeley Springs. In the foreground: Mahantango, Marcellus and Needmore shales (Devonian). In the background: Oriskany (Devonian; lower ridge) and Tuscarora (Silurian; main ridge) sandstones. Photo: J. Donovan

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AUTHORS

Cristine M. Vinciguerra � Departmentof Geology and Geography, West Virginia Uni-versity, 330 Brooks Hall, Morgantown, WestVirginia 26506-6300; present address: AECOM,40 British American Boulevard, Latham, NewYork 12110,

Cristine M. Vinciguerra is a hydrogeologist atAECOM in Latham, New York. She has a master‘sdegree in Geology from West Virginia University.Her interests include groundwater and surface-

Spatial variations in apparentrecharge rate to a montanePaleozoic bedrock aquifer,Appalachian Mountains,United StatesCristine M. Vinciguerra and Joseph J. Donovan

water hydrology of mountainous karst terranes.

Joseph J. Donovan � Department of Geol-ogy and Geography, West Virginia University,330 Brooks Hall, Morgantown, West Virginia26506-6300; [email protected]

Joseph J. Donovan teaches groundwater scienceat West Virginia University and performs re-search in recharge processes, mining hydroge-ology, and Holocene paleoclimate of the NorthernGreat Plains.

ACKNOWLEDGEMENTS

This project was supported by the West VirginiaBureau for Public Health; the Morgan County(West Virginia) Rural Water Committee; the U.S.Department of Agriculture Cooperative StateResearch, Education, and Extension Service.Jack Soronen piloted the aircraft used forphotography in Figure 4. Field assistance wasprovided by Matt Finkenbinder and Kevin Rega.Helpful ideas and advice were offered by EbWerner and Annie Morris.

ABSTRACT

Hydrograph separation methods for estimating recharge are not use-ful in all watersheds, but in this article, the hypothesis is tested thatthey should be reasonably accurate in montane humid locationswhere dissipation times for runoff are short. The study area is a mon-tane carbonate/clastic Paleozoic aquifer in the Appalachian Moun-tains, West Virginia and Virginia. For 1 yr, groundwater dischargewas continuously measured at three different stream reaches; eachhas grossly similar climate, soils, and topography, but differing ge-ology underlying the main stream channels and different stream ori-entations with respect to structure. One station measured base flowin a strike-parallel stream of moderate length (13.4 km [8.3 mi]);the other two in strike-normal streams of shorter length (2.7 km[1.7 mi]). Pronounced differences were observed in apparent re-charge rate between the strike-normal and strike-parallel streams,with the strike-parallel stream similar in apparent recharge to 21other regional streams, with both large and small catchments, ofsimilar climatic and geologic conditions. Similarly, the strike-normalstreams in the study area were significantly lower in recharge com-pared with these. The precise location of stream losses, if any, couldnot be pinpointed in detailed streamflow measurements. Althoughthe low apparent recharge rates are primarily ascribed to streamlosses, underflow, and/or hyporheic interactions, the evidence forthis was nonconclusive. This case underscores that even in humidmontane settings of moderate relief, complications in the ground-water budget may arise that make recharge estimates based on sur-face hydrograph analysis inaccurate.

Copyright ©2010. The American Association of Petroleum Geologists/Division of EnvironmentalGeosciences. All rights reserved.DOI:10.1306/eg.04061010001

Environmental Geosciences, v. 17, no. 3 (September 2010), pp. 121– 136 121

INTRODUCTION

Because of the wide availability of streamflow data, re-charge rates are commonly estimated from continu-ously recorded streamflow observations (Rutledge andDaniel, 1994; Mau and Winter, 1997; Rutledge, 1998;Kozar and Mathes, 2001). Meyboom (1961) was thefirst to describe the log-linear relationship that com-monly prevails between groundwater discharge andtime. The technique implicitly assumes approximateequivalence between groundwater recharge and ground-water discharge (i.e., base flow) plus or minus changesin storage.Where the techniquemay be applied, stream-flow is primarily sustained by groundwater dischargeeven during extended periods without precipitation(Sophocleous, 2002). Time integration of this base flowdischarge serves as a proxy for recharge to the aquifer.To determine the base flow component of streamflow,some type of hydrograph separation must be per-formed, different methods for which are outlined byMeyboom (1961), Nathan and McMahon (1990), andRutledge andDaniel (1994). Apopular hydrograph sep-aration technique is the recession-curve-displacementmethod (Rorabaugh, 1964), an automated version ofwhich RORA (Rutledge, 1998) sums upward shifts inthe streamflow recession curve that occur following re-charge events.

Hydrograph separation does not specifically ac-count for other terms in the groundwater budget, suchas evapotranspiration, pumping abstraction, and under-flow (Rutledge, 1998).Without compensation for suchfluxes, resulting cumulative base flow might be moreaccurately termed “apparent recharge,” with referenceto the approximate equivalence between groundwaterrecharge and base flow. The method may be applied tospringflows as well, provided that the area of ground-water capture can be estimated (Ketchum et al., 2000).

The variety of complications thatmay result from ap-plication of the technique has been reviewed byHalfordand Mayer (2000), Scanlon et al. (2002), Sophocleous(2004), and others. First, the groundwater contributionto apparent rechargemust also be estimated during andimmediately after precipitation events because stream-flow includes both groundwater discharge and runoff inthese periods and separation can be ambiguous. This sep-aration is much more difficult if precipitation events arefrequent, streams are regulated, the basin is large, and/or the measuring station is relatively far downstream(Scanlon et al., 2002). Second, the area of the contrib-uting aquifer must be estimated for use in calculating

122 Spatial Variations in Apparent Recharge Rate to a Montane

spatial-averaged apparent recharge rate (Rutledge,1998); commonly, it is assumed to correspond to sur-face watershed topography. Third, when evapotranspi-ration, underflow, and abstraction are not negligiblysmall, they must be estimated and compensated for.Losing stream reaches are also problematic, as are anyhydrogeologic settingswithinwhich stream losses or hy-porheic-zone interaction occurs (Scanlon et al., 2002).

As a result of such problems, the viability of apply-ing hydrograph separation techniques to recharge esti-mation has been questioned. Although favored inpractice bymany, hydrograph separation to estimate re-charge has been described as “an act of desperation thatneed not occur” (Halford, 2008). The techniques maybe easily misapplied in low-relief or excessively largewatersheds within which surface runoff duration orgroundwater catchment areas are difficult to define(Rutledge, 2008). Even practitioners of the techniqueallow that, in some types of drainages, the results maybe unpredictably erroneous (Rutledge, 2008). For thetechnique to be viable, a particular ground watershedmust be a priori deemed suitable for such analysis.The question arises, “Are there some watersheds thatare naturally suitable for application of the method,and others that are not?”

It may be hypothesized that one type of watershed,at least, in which the method should be generally ap-plicable exists: small montane basins of headwaterstreams in humid climates. Streams in such basins arenormally observed to be gaining and their hydrographsrelatively easily separable into flow components be-cause of rapid transit of storm peaks past points of flowmeasurement. In this type of setting, base flow wouldideally tend to dominate streamflow soon after precipi-tation events and without masking by runoff. Stream-flow data, therefore, from such watersheds wouldnormally be amenable to application of hydrograph sep-aration techniques. Hydrograph-based estimation ofgroundwater recharge for small mountain watershedshas yielded generally favorable results (Mau andWinter,1997; Kozar and Mathes, 2001).

Purpose

The purpose of this study was to examine the coher-ence of conventional streamflow-based recharge esti-mates at three different locations within a montanestudy area of reasonably uniform geology and climate.The approach will be to compare apparent recharge

Paleozoic Bedrock Aquifer, Appalachian Mountains

rates based on hydrographmethods along three streamsdraining a similar Paleozoic aquifer sequence in theVal-ley and Ridge physiographic province (Fenneman,1928), Appalachian Mountains, United States. The

three streams (from south to north: Breakneck Run,Rock Gap Run, and Sir Johns Run, which will be abbre-viated as BR, RGR, and SJR, respectively; Figure 1) arelocated along geologic strike with respect to each other

Figure 1. Location map of the CacaponMountain aquifer. Triangles indicate streamgages for this study. PA = Pennsylvania;MD = Maryland; WV = West Virginia; VA =Virginia.

Vinciguerra and Donovan 123

in a sequence of folded Devonian-Silurian sedimentaryrocks. Both climate and surface water flows within thisarea are grossly homogenous, as are topography and


soils. However, differences in the geomorphic and struc-tural settings between each stream exist, which may in-fluence their apparent recharge rates. This rural area is

Figure 2. Geology of Cacapon Mountainaquifer, with gaging station locations(circles). Cross section A-A′ refers to Figure 3.Geology from Cardwell et al. (1968).


relatively undeveloped by man, so anthropogenic influ-ences are not thought to be an influencing factor.

Objectives were:

1. to estimate apparent recharge from cumulative baseflow in these three small catchments over a singlebedrock aquifer for a reference period of 1 yr,

2. to test the hypothesis that these recharge rates werehomogenous for these drainages, and

3. to test the hypothesis that these rates were not sig-nificantly different from those within geologicallysimilar settings within the same region.

Study Area

The study area is theCacaponMountain aquifer (Figure 1),which includes a sequence of bedrock strata betweenthe Tuscarora Formation (Silurian) and the OriskanyFormation (Lower Devonian). The upper half of the se-quence is composed mainly of carbonate or calcareous-cemented rocks; rocks in the lower half of the sequenceare dominantly clastic. In both clastic and carbonateunits, fracture porosity tends to dominate matrix poros-ity. Warm Springs Ridge (underlain by Oriskany)bounds the aquifer on the east, and the crest of the Ca-caponMountain anticline, where Tuscarora quartzite isexposed, bounds it to the west. The aquifer extendsnorth to the Potomac River, where it plunges north

under Devonian cover, and south to near the Virginia–West Virginia border, where a mapped fault is thoughtto represent a local barrier to groundwater flow. Theportion of the aquifer studied here lies on the east sideof the Cacapon Mountain anticline, forming an elon-gate (3 × 40 km [3 × 25 mi]) sequence following thenortheast–southwest strike of the folded Valley andRidge province in this area. Surface drainage within thisfolded sedimentary sequence tends to have trellis form.

Figure 2 shows the geology of the study area fromthe regional-scale mapping of Cardwell et al. (1968).Figure 3 is a schematic cross section of the east flank ofCacaponMountain, showing approximate stratigraphicrelationships at depth across section A-A′ in Figure 2.This is a typical Valley and Ridge anticlinal mountain,with strike-parallel ridges and lowland valleys formedby alternating resistant and nonresistant lithologies.The large-scale structure of the province is dominatedby imbricate thrust and reverse faulting and folding,which mostly occurred during the Pennsylvanian-ageAlleghanian Orogeny (Kulander et al., 1995). Any flowof groundwater perpendicular to strike within Valleyand Ridge-type structures is generally restricted bystrike-parallel ridges of less permeable units (Swainet al., 1991). On Cacapon Mountain, such aquitardunits include the Tuscarora Fm. (cemented ortho-quartzite) at its base and, locally at least, the OriskanyFm. (fractured but well-cemented sandstone) along its

Figure 3. Schematic cross section (not to scale) of subsurface geology and hydrogeology along section A-A′ (Figure 2).


outer flanks. Although large amounts of water are beingwithdrawn from different aquifers in the Valley andRidge, recharge processes that occur in these settingsare still poorly documented (Rutledge and Mesko,1996).

The geology of the eastern side of the CacaponMountain anticline in the study area is well expressedtopographically (Figure 4). The anticline in this areais slightly north plunging toward the Potomac. From old-est to youngest, the major rock units involved (fromleft to right in Figure 4) are the Tuscarora Fm., ClintonGp., McKenzie Fm., Wills Creek Fm., TonolowayFm., Helderberg Fm., and Oriskany Fm. The Silurian-Devonian transition occurs within the Helderberg Fm.The central ridge former is the Tuscarora, a dense ce-mented orthoquartzite 60 m (197 ft) thick along thefold axis. The Helderberg limestone is in most locationsthe most transmissive rock unit in this sequence and hasa tendency to form karst conduits and caves (Kulanderet al., 1995), several of which are observed in the studyarea. The drainage patterns of streams in the study areaare of typical trellis form. Sir Johns Run, a tributary of thePotomac, flows strike-parallel within the topographicdepression between the Tuscarora and Oriskany ridges,whereas BR and RGR, tributaries to Sleepy Creek, flowdominantly strike-normal through water gaps in theOriskany (Figures 1, 2). These three catchments are allsubject to the same climate, receive similar amounts ofrainfall, and overly the same aquifer units, with one ex-ception: RGR and BR cross the subcrops of all forma-


tions in the sequence, whereas SJR only flows overrocks upsection as far as the Tonoloway Fm. The SJRis also a larger watershed (13.4 km [8.3 mi] streamlength) than either RGR (2.7 km [1.7 mi]) or BR(2.7 km [1.7 mi]). Finally, the headwaters of all threestreams extend to the higher elevations of CacaponMountain (∼650 m [∼2100 ft] above sea level), butSJR extends to lower elevation (∼150 m [∼490 ft])than the other two (∼240–260 m [∼790–850 ft]).

The geology underlying the three watersheds dif-fers only in detail. The SJR flows almost completely ina strike-parallel orientation across Tonoloway andWillsCreek formations (limestone) in its upper reaches and, far-ther downstream, across Clinton-MacKenzie clastic units.Over its entire reach, it flows within ±20° of bedrockstrike. The RGR and BR, however, cross all these forma-tions as well as the Helderberg and Oriskany in a more-or-less strike-normal orientation, with minor or obliquestrike-parallel components. In the headwater reaches ofall three streams, but especially RGRandBR, unmappedQuaternary boulder deposits cover the lower slopes ofCacapon Mountain and overly strata (Figure 3). Thesecolluvial deposits act as very shallow unconfined aqui-fers that intercept and temporarily store infiltration.Sandstone cobbles and boulders abundant in these de-posits are derived from either the Tuscarora or Clintonstrata (Vinciguerra, 2008). Little is known about thethickness or detailed distribution of these colluvial grav-els as they have not been included as map units on geo-logic maps of this area, but they are thought to be

Figure 4. Air photograph looking northfrom between BR and RGR stream stations,showing geology. Notation as for Figures 1and 2.


relatively thin (<5 m [16.4 ft]). They serve as the im-mediate source for base flow and springs in the upperreaches of these streams.

METHODOLOGY

The three gaging stations were installed using datalogger-coupled nonvented 13-ft (3.9 m) range pres-sure transducers (Onset Computer model U20-001)at downstream locations (along RGR, SJR, and BR;Figure 2). Stilling wells (10-cm [3.9-in.] diameterschedule 40 polyvinyl chloride pipe) were installed ad-

jacent to stream reaches containing pools with a mini-mum of large boulders. These included a horizontalportion of slotted pipe beneath the creek bed belowminimum stage and a vertical portion cut a meter intothe streambank, used to house the suspended transducer.Transducers accurately measured stage in the stillingbasins at hourly intervals. A sealed Solinst BaroLoggerwas used to measure atmospheric pressure variationsat the BR station around a zero datum of 93.148 kPa(13.5 psi). Barometric data were used to correct all thesealed water pressure loggers for atmospheric pressurefluctuations. Resulting stream stage measurementswere recorded to 0.001 m (0.003 ft) resolution and arethought to be accurate to 0.01 m (0.03 ft) or better.

Rating curves were developed from a minimum of10 manual flow measurements taken throughout2006–2007 at each of the three stations. Flowwas mea-sured by standard cross-channel profiling, taking amini-mum of 12 current-velocity measurements across eachchannel section at 15 cm (6 in.) horizontal intervals andat a water depth 0.6 times the total depth. Instrumenta-tion was a pygmy-type magnetic-rotor flowmeter on astandard 1.2-m (3.9 ft) U.S. Geological Survey wadingrod and integrated for 40 s using an AquaCalc data stor-age/integration meter. Replication of profiles was notroutinely performed, but replication experiments onselected profiles suggested approximate standard errorsof ±11% (n = 4) in channels of this form with a singleoperator. Figure 5 shows rating curves of stage versusflow at each site, with an estimated 20% uncertaintyin each measurement. Least-square fits to these dataof either exponential or polynomial form were usedto optimize correlation between the rating curve andthe measurements; values of R2 greater than 0.97 wereobtained at all stations. The resulting rating modelswere used to convert stage to flow. Like most ratingcurves, this conversion is most accurate at lower flowsand most uncertain at higher flows.

Rated total stream discharges for each locationwere calculated on an hourly interval and summed todaily flows by midpoint integration. Calculation of ap-parent recharge requires separation of the base flowfrom runoff components, which was performed byboth direct (visual) and automated techniques. Directseparation followed the following steps:

1. In a plot of hourly log discharge versus time, straightline (i.e., log-linear) segments were fitted to both thesteepest early recession limb (interpreted as storm run-off) and the shallowest late-recession limb (interpreted

Figure 5. Rating curves for Sir Johns Run (SJR), Rock Gap Run(RGR), and Breakneck Run (BR).


as base flow). In addition, an intermediate recessionslope—steeper than that of base flow—was fittedwhere present and interpreted as quickflow dis-charge from fast pipe flow in large conduits (e.g.,Long, 2009, p. 250). Base flow is assumed to repre-sent discharge from primarily matrix flow within theaquifer, inasmuch as overland flow in karst aquifersis generally minor. Base flow was back extrapolatedto the start of the recharge event and used for differ-entiating between the other line segments for calcu-lation of runoff and quickflow.

2. From the beginning of the recharge event, base flowwas antilogged and integrated, then used to calculaterunoff by difference with total flow.

3. Integration of base flow was performed during theperiod of record and normalized to drainage areato estimate apparent recharge rate.

By convention, the visual method excluded runofffrom estimated recharge but included the sum of baseflow and quickflow. Precise discrimination into runoff,quickflow, and base flow (slow flow) based on hydro-graph observations alone is well known to be subjectivebut is nonetheless widely practiced in karst discharges(Sloto and Crouse, 1996). The time used for cessationof runoff was by convention designated at the intersec-tion of log-linear slopes for runoff and base flow or (ifpresent) quickflow. Although an unworkable techniquefor gaging stations located far downstream, at theseheadwater locations, nearly all runoff passes the gageswithin only a few hours after cessation of precipitation;for sudden storms in this study, storm hydrographspeaked in all three stationswithin 6 hr and had fully sub-sided by 40 hr. Complexities in separation occurred inperiods of multiple consecutive storm events or longcontinuous rainy periods, of which two were presentin this study. These are not thought to have added sig-nificant error to the recharge estimate.

In addition to the log-linear separation, automated(computer-based) separation RORA (Rutledge, 1998,2000, 2007) was performed. Results were obtained inall cases that were consistent with the visual separationresults within ±15%. The RORA results commonly un-derestimate apparent recharge (Rutledge, 2007).

The apparent recharge rate estimates from the Ca-caponMountain aquifer were also compared with ratescalculated for nearby locations in the lower Paleozoic intheValley andRidge province. Flow datawere obtainedfrom the U.S. Geological Survey National Water Infor-mation System database. Because only daily average


discharges were available, these data from July 2006to July 2007 (the index period for this study) were usedto estimate recharge using RORA, without visual sep-aration. Inputs for RORA include recession index, cal-culated using RECESS (Rutledge, 1998), drainage area,and daily discharge measurements.

To further examine the possibility of local streamlosses or hyporheic interaction in the Cacapon studyarea, synoptic flow measurements were performed atseveral stations along RGR (a strike-normal stream)to observe spatial patterns of losses and gains. The mea-surements were done on two different dates (April 25,2007, and April 17, 2008), with the second date cho-sen to have surface-water flows highly similar to themeasured 2007 flows. In replicate measurements atfour stream profile locations, the differences in flow be-tween the two dates averaged 7% of total flow at eachstation, within estimated measurement uncertainty forthe open-channel technique. Therefore, it was consid-ered reasonable to consider the two sets of measure-ments together. Both surveys were done with a pygmy-style flowmeter and an AquaCalc instrument as for thestreamflow rating.

In 2006–2007, RGR had extremely low flows insummer (<0.02 m3/s). All rated flows below 0.01 m3/s(the smallest flow measurable with a flowmeter on thisreach) were not considered reliable and were desig-nated as below detection.

A limitation of this study is the short duration ofcontinuous streamflow data. This was dictated in partby vandalism of the SJR streamflow station in Fall2007.

RESULTS

Streamflow Measurement

Figure 6 shows streamflow data for stations SJR, BR, andRJR for July 1, 2006, through July 1, 2007. Also shownare rated flows for Tonoloway Spring, a limestone-hosted spring within the Rock Gap Run drainage up-stream from the RGR station (Figure 1). It occurs nearthe contact between the Helderberg and Tonolowayformations and discharges into RGR upstream of thegage. The brief downward "spikes" in discharge are dailydrops in stage when water was diverted from the springenclosure to fill water trucks. With the exception ofbrief and abrupt increases in spring discharge periodsafter major storms, Tonoloway Spring is an excellent


measure of pure base flow. The downward deflectionsin the Tonoloway hydrograph are intermittent periodsof commercial abstraction of water from the holding res-ervoir in which the stage was measured for flow estima-tion. These do not represent actual flow declines.

Both BR and SJR flows are similar in form to eachother, with a major difference in flow because of a dif-ference in groundwater catchment area. TheRGR, how-ever, shows steeper log-normal recession and, whileshowing higher flows during the recharge freshet of Feb-ruary throughApril, quickly lost flowwithin amonth ormore without recharge. No known abstraction for anybut low-volume rural residential use is known for RGRor any of the other drainages.

Estimation of Apparent Recharge

Table 1 shows variation in interpreted runoff, quick-flow, and base flow for the three watersheds. TheSJR and BR are both dominantly base flow (53% and79%, respectively). The RGR, however, is runoff domi-nated (55%). Quickflow was a relatively minor compo-nent in all three streams. On average for these streams,all of the storm runoff has passed the gaging stationwithin 35 to 40 hr after a single precipitation event.

In 2006–2007, apparent recharge rate was substan-tially higher for the strike-parallel watershed comparedwith the two strike-normal reaches (Table 2). Esti-mated recharge rates by visual separation and by RORAanalysis both give comparable results for BR and SJR(within 14%) but gave a much larger discrepancy forRGR (6.40 cm [2.5 in.]/yr visual vs. 9.32 cm [3.67 in.]/yr RORA). The reason for this large percentage discrep-

ancy is not clearly known but may relate to the rela-tively lower discharge at this station.

The extremely low values at RGR and BR promptcomparison to apparent recharge rates for other streamsof similar geology within the same region. A sample of21 other streams in the Valley and Ridge physiographicprovince of the Appalachian Mountains was selectedfor analysis of apparent recharge during the same peri-od (Figure 7; Table 3). Stations were selected based on(1) location within the Valley and Ridge, (2) sourcebedrock of lower Paleozoic (Devonian–Cambrian),and (3) availability of at least 25 yr of record to assurethat 2006–2007 results might be compared with thelonger term record for consistency. All have trellis drain-age characteristics, and most are dominated by strike-parallel reaches, with a few exceptions. Daily averagedischarge data from these stations between July 2006and July 2007 were used to estimate apparent rechargeusing RORA, by the same methodology as applied tothe CacaponMountain streams. Results include spatial-ly normalized (areal) apparent recharge rate, expressed

Figure 6. Streamflow hydrographs (leftscale) for the three continuous stationsduring the study period, as well as flowsfrom Tonoloway Spring (top; right scale).

Table 1. Relative Proportions of Flow Components for SJR, BR,
and RGR Hydrograph Separations
SJR∗

Vinciguerra and

BR∗∗

Donovan

RGR∗∗

Base flow
53.2 79.0 29.1 Quickflow 18.7 10.3 15.9 Runoff 28.1 10.7 55.0
Values are presented as percent of total annual flow.∗Strike-parallel.∗∗Strike-normal.

129

as a proportion of local precipitation; recession index(calculated by RECESS); average precipitation; ele-vation above sea level; and reach geology (Table 3).This set of streams shows a bimodal variety in catch-ment size and in general fall into two groups: large(>285 km2 [110 mi2]; n = 14) and small (<50 km2

[19.3 mi2]; n = 7). Even within the subset of smallstreams, all are somewhat larger in size than the Caca-pon Mountain drainages. Most streams in this regionaldata set have gaging stations located closer to their


mouth instead of in the montane headwaters as in thisstudy.

Despite the wide range of catchment sizes, littlestatistical difference exists between the mean normal-ized recharge rates of these regional streams, large orsmall (Table 3). The mean values of apparent rechargefor these regional streams are compared statisticallywith those of the Cacapon Mountain data set (Table 4).T tests assuming unequal variance were applied usingall three watersheds in the Cacapon set and only thestrike-normal watersheds. Because no statistically sig-nificant difference exists between mean apparent re-charge for the small and large subsets of the regionalstreams (Table 4), comparison was made to the smallregional subset only. The null hypothesis was testedthat no difference exists between the means of thesmall-area regional subset and the Cacapon Mountainstreams. The means were significantly different be-tween the strike-normal Cacapon and small regionalstreams at the 97.7% confidence level, but not signifi-cantly different (88.0% probability) if SJR was included

Table 2. Estimated Apparent Recharge Rates for 2006–2007 in
the Cacapon Mountain Streams
Method
SJR∗ BR∗∗ RGR∗∗
Visual
33.8 16.7 6.40 RORA 33.4 19.1 9.32
Values are presented as centimeters per year.∗Strike-parallel.∗∗Strike-normal.

Figure 7. Streams in the Val-ley and Ridge physiographicprovince used for statisticalcomparison.


in the Cacapon mean (Table 4). Both RGR and BR fallwell outside the 95% confidence limits on the regionalmean, whereas SJR falls within these limits (Figure 8).It may be concluded that for 2006–2007, SJR was notsignificantly different from other regional streams,whereas RGR and BR both were. Thus, in comparisonwith regional streams of similar climatic and geologicconditions, but of somewhat larger catchment size, therates of apparent recharge for Cacapon strike-normalstreams are clearly anomalously low.

Synoptic Observations of Flow

Results of the synoptic flow measurements in RGR areplotted spatially on Figure 9, with triangles denoting the2007 measurements and dotted circles denoting thosemeasured in 2008. The elevation difference betweenthe first and final measuring point is 131 m (430 ft).

Flows are first observed below high-elevationsprings near the break in slope below the steepest por-tion of the mountain corresponding to the Tuscarora-Mackenzie contact (Figure 9). Moving downstreamfrom these springs, both springs and sinks were ob-served over the bouldery deposits blanketing the slope(within the Sm and Sc units on Figure 9), but no sub-stantial net losses or gains were measured; this intervalshows a high degree of hyporheic interaction, and rep-resentative flow measurements could not be collected.Flows substantially increased and became less influ-enced by hyporheic interactions at the contact betweenMackenzie clastics and Tonoloway/Wills Creek carbon-ates, which also represented a second change in slope.From this point downstream, flows were measurableand indicated gaining stream conditions in downstreamdirection (Figure 9). Within the Oriskany-Helderbergwater gap (within which station RGR lies), flows leveloff at a maximum of 141 L/s, with locally lower values.Downstream of the RGR station at the lower end of thewater gap to where RGR enters Sleepy Creek, little var-iation in flow (131–140 L/s) exists (Figure 9). This sug-gests that at least in this level-slope part of the stream,discharge of groundwater to the stream as base flow hadmostly diminished by late April.

These detailed flow measurement results offer in-conclusive evidence as to the nature of hypothesizedreduced recharge rate measured for RGR. The stream-flow variations do not clearly indicate any pronouncedsinking reach, similar to a sinkhole. They do show somelocal variability in flow within the water gap as well asbetween stratigraphic units; local hyporheic variability

in streamflow over both limestone and bouldery units,and some correlation between streamflow and eleva-tion. In contrast, downstream of the Silurian–LowerDevonian limestone/sandstone sequence, the flowsover Devonian shale were nearly uniform and showedlittle downstream variation, as either gains or losses,and no hyporheic variability.

DISCUSSION AND SUMMARY

In the study area, two strike-normal drainages (BR andRGR) and one strike-parallel drainage (SJR) in a mon-tane setting were analyzed for surface flow rates duringa 1-yr period (2006–2007) to calculate apparent re-charge rates upstream of these stations. The two strike-normal drainages averaged significantly lower apparentrecharge in comparison with the strike-parallel drain-age; the rapid rate of recession in RGR was reminiscentof a losing stream reach. Four factors that could poten-tially account for the large observed difference include:

1. differences in riparian evapotranspiration flux,2. a fundamental difference in spatial recharge rates

between catchments,3. differences in groundwater abstraction/pumping

that might result in substantial differences in the lo-cations of groundwater and surface water divides, or

4. differences in streamflow losses, for example, eitherhyporheic flow or underflow to deeper aquifers.

Analysis of digital elevation models and aerial pho-tographs suggests that the riparian areas do not extendmore than 5 to 20 m (16.4–65.6 ft) from each streamand do not differ greatly from one catchment to another(Vinciguerra, 2008). The three streams in this study allhave very similar vegetation and water table depth.Therefore, large variations in evapotranspiration rateare not considered likely, and factor (1) is thought tohave a minor influence on the water budget.

Similarly, factor (3) (abstraction) is thought to beunimportant in this case.Only low-yield and low-densityresidential well abstraction occurs.

The topography, elevation hypsography, and soilsfor the three reaches are virtually identical and wouldnot be likely to be a source of variation in spatial re-charge rates. Steep slopes in all three catchments arepresent, but not to a significantly different degree fromeach other. It is possible that there could be some un-certainty in calculation of groundwater catchment area


Table 3. Hydrologic and Geologic Characteristics of Regional Streams in the Valley and Ridge Region Compared with Study Area Streams

Stream Gaging Station

132 Spatial Variations in

DrainageArea(km2)

Apparent R

RecessionIndex

(d/log cycle)

echarge Rate

EstimatedPrecipitation(cm/yr)

to a Montane

2006–2007Recharge(cm/yr)

Paleozoic B

NormalizedRecharge

(% of precipitation)

edrock Aquifer, Appal

GageElevation(m)

achian Mo

BedrockGeology

Cacapon River at Great Cacapon,West Virginia

1755.0
25.6 98.8 21.2 21.5 137.0 Silurian–Devonian
Patterson Creek near Headsville,West Virginia

548.6
Back Creek near Jones Spring,West Virginia

611.0
Opequon Creek nearMartinsburg, West Virginia

709.8
40.8 100.1 27.3 27.3 106.5 C ambrian–Ordovician
S. Fork S. Branch Potomac R.,Moorefield, West Virginia

720.2
Waites Run near Wardensville,West Virginia

30.2
Marsh Run at Grimes, Maryland
49.1 63.5 100.2 20.1 20.1 106.4 C ambrian–Ordovician Antietam Creek nearSharpsburg, Maryland
730.6
Conococheague Creek atFairview Maryland

1284.4
Tonoloway Creek nearNeedmore, Pennsylvania

27.8
Aughwick Creek near ThreeSprings, Pennsylvania

533.0
Sherman Creek at ShermansDale, Pennsylvania

520.0
Kishacoquillas Creek atReedsville, Pennsylvania

426.4
Bixler Run near Loysville,Pennsylvania

39.0
Yellow Breeches Creek nearCamp Hill, Pennsylvania

561.6
Bullpasture River at Williamsville,Virginia

286.0
Jackson River near Bacova,Virginia

408.2
North River near Stokesville,Virginia

45.0
Muddy Creek at Mount Clinton,Virginia

37.2
Hogue Creek near Hayfield,Virginia

41.3
Potts Creek near Covington,Virginia

397.8
Mean of regional streams
464.9 32.9 98.4 30.4 30.9 239.6 Mean of streams <40 km2 38.5 31.1 95.5 31.1 33.0 297.2
untains

based on surface drainage divides. However, this error isthought to be too small to greatly influence resulting rates.

The most outstanding differences between thestrike-normal and strike-parallel drainages are geologyunderlying the main channel. The channel of SJR is un-derlain by relatively little limestone and only in theupper 3 km (1.9 mi) of its reach; this includes theWillsCreek Fm., the lower part of the Tonoloway Fm., andnone of the Helderberg or Oriskany fms. Over most(16 km [10 mi]) of its length, it flows over lower Silu-rian Clinton-McKenzie clastic rocks. Its channel is theonly one of the three studied that does not flow directlyover the Helderberg limestone/Oriskany sandstone.The Helderberg has been extensively reported as aconduit-forming unit (Kulander et al., 1995) and is, insome locations, a cave former (Price andLudlum, 1951).Large openings at the surface were observed near theHelderberg-Tonoloway contact in three locations with-

in the study area, including one shown on Figure 9. As aresult, the potential for losses to or gains from this for-mation is thought to be higher than for other forma-tions. Although no streamflow measurements weremade along the channel of BR, its geology is a closematch for that of RGR and one ephemerally discharg-ing cave and a conduit spring within the Helderberg-Tonoloway are observed there (Vinciguerra, 2008).Both streams, therefore, would seem to have the poten-tial for stream losses (or gains) associated with this strat-igraphic interval.

The question arises as to, if such losses are occurringand are the cause of the reduced apparent recharge,where the lost surface water is being directed. Waterlosses on RGR clearly do not appear to resurface furtherdownstream of the gaging station upstream of its con-fluence with Sleepy Creek, suggesting that these arenot simply hyporheic losses. Another possibility is that

Table 3. Continued

Stream Gaging Station

DrainageArea(km2)

RecessionIndex

(d/log cycle)P
Estimatedrecipitation(cm/yr)
2
006–2007Recharge(cm/yr)
NormalizedRecharge

(% of precipitation)

Vincigue

GageElevation(m)

rra and D

BedrockGeology

SJR, this study
21.5 26.5 96.7 33.4 34.6 124.8 Silurian–Devonian RGR, this study 10.9 20.7 96.7 9.3 9.6 222.0 Silurian–Devonian BR, this study 6.6 61.9 96.7 19.1 19.7 274.8 Silurian–Devonian
Recharge rates calculated using RORA.

Table 4. Statistical Comparison of Apparent Recharge Between Cacapon Mountain and Regional Stream Gages for 2006–2007

Cacapon Mountain
Regional
All
Strike normal All >50 km2 <50 km2
n
3 2 21 14 7 Mean, % 21.3 14.7 30.9 29.9 33.0 Standard deviation, % 12.5 7.1 9.5 5.8 14.9 Standard error, % 7.2 5.0 2.1 1.5 5.6 Coefficient of variation, % 0.59 0.48 0.31 0.19 0.45 Minimum, % 9.6 9.6 20.1 21.5 20.1 Maximum, % 34.6 19.7 58.5 40.6 58.5
mA
mB T df Probability of significance,mA < mB (one-tailed), %
Regional <50 km2
Regional >50 km2 0.53 19 30.2 Cacapon (all) Regional <50 km2 1.27 8 88.0 Cacapon (strike normal) Regional <50 km2 2.43 7 97.7
onovan 133

stream losses contribute to deeper underflow, discharg-ing to the main Sleepy Creek channel or to deeper arte-sian units. No direct evidence exists for or against deep


underflow losses. However, if neither ET losses, funda-mentally lower recharge rates, or abstraction differencesare occurring, anunderestimationcausedby stream losseswould be the most likely interpretation.

Although the possibility of deep underflow is spec-ulative, it raises intriguing possibilities. Certain springsin the Valley and Ridge have been suspected to have adeep thermal origin, includingBerkeley Springs (Figures 1,2) about 14 km (8.7 mi) north of RGR. BerkeleySprings is 11°C warmer than average shallow ground-water temperature, which suggests a minimum circu-lation depth of 300 m (984 ft) (Boughton and McCoy,2006) and a deep convective origin. Hobba et al. (1979)speculated that flow paths from 250 to 1600 m (820–5250 ft) in the Oriskany Fm. could account for theanomalously high thermal signature of water in Berke-ley Springs and others within the Appalachians, occur-ring associated with the crests or limbs of anticlines.The chemistry of spring water at Berkeley Springs is vir-tually identical with the chemistry of springs withinRGR, including Tonoloway Spring (Figure 1) (Corder,2008). Although it can be by no means considered

Figure 8. Absolute frequency histogram of normalized appar-ent recharge, as a percentage of local precipitation, for the regio-nal stream data set (red bars) and Cacapon Mountain streams(blue-gray bars).

Figure 9. Streamflow (liters per second)along Rock Gap Run (RGR) on April 25,2007 (triangles), and April 18, 2008(dotted circles). Station RGR for apparentrecharge measurement is circled. Geol-ogy from Cardwell et al. (1968).

Paleozoic Bedrock Aq
uifer, Appalachian Mountains

demonstrated, stream losses of the type indicated atBR and RGR could contribute to such a deep ground-water flow system.

The fact that the strike-normal streams in thisstudy cross the structural front of Cacapon Mountainmay be a key to the suspected stream losses. Wilsonand Guan (2004) describe the mountain front as theinterface between the mountain block and the struc-tural basin within block-faulted crustal elements andconsider these as ideal locations for deep-crustal rechargein semiarid climates. Although the structural style ofthis study is distinctly different from the normal-faultedsemiarid basins to which these authors allude, here,compressive structures in conjunction with fold andstream geometry may interact to give similar potentialsfor stream losses and deep-aquifer recharge.

In summary:

1. Two stream reaches in this study (RGR and BR) hadmuch lower apparent recharge rates than a thirdstream (SJR). Although this is a very small sample,it is proposed that apparent recharge rates of strike-parallel and strike-normal streams of this study differsystematically, with the strike-parallel stream (SJR)showing higher apparent recharge rates. Normalizedrecharge rates from the strike-normal streams alsodiffer significantly from a sample of strike-parallelstreams in the Valley and Ridge region.

2. These results are in agreement with other investiga-tions in karst terranes (Padilla et al., 1994; Lee andLee, 2000) that base flow can exert varying degreesof control over surface water and spring flows at dif-fering locations of measurement. The implication isthat estimating recharge rates from streamflow datamay give highly inaccurate results if the stream reachstudied flows over transmissive conduit-forming lime-stone or extensively fractured strata. Stream lossesupstream of the gaging station will generally causelower recharge rate estimates and the converse forgaining reaches. These outcomes are not surprisingper se, but unfortunately, identification of gainingor losing reaches lacks formal protocol and is not al-ways performed when using available surface-waterflows for recharge estimation.

3. Although small mountain watersheds in humid cli-mates appear ideal for the application of hydrographseparation methods to estimate recharge, measure-ments should be made upstream of gaging locationsto ensure that losing or gaining reaches do not occur.This is particularly true where conduit-prone lime-

stone formations outcrop. However, the postmea-surement survey of upstream streamflows for RGRin this study showed local streamflow variability,but no glaringly obvious losing reaches that would al-low anticipation of losses to underflow. Therefore, itcan probably not be presumed, in general, that losingstream reaches will be straightforward to identify, asthey are in many cases (e.g., swallow holes).

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