The Influence of Well Construction on Groundwater Flow in Columbia River Basalt – Examples from Mosier, Oregon

by

Kenneth E. Lite Jr. Oregon Water Resources Department

Purpose

• Demonstrate why the geologic framework matters in the Mosier groundwater flow system.

• Show examples of commingling wells.

• Show how commingling wells have affected the Mosier groundwater flow system.

Geology of the

Columbia River Basalt Group

Mosier Area

Surface Geology

B

B’

Dalles Formation Pomona Selah interbed Priest Rapids Frenchman Springs Grande Ronde

Columbia River Basalt Group

Dalles Formation

Pomona Basalt

Rosalia Flow: Priest Rapids Basalt

Roza Basalt

Frenchman Springs Basalt

Lolo Flow: Priest Rapids Basalt

Dalles Formation

Pomona Basalt

Rosalia Flow: Priest Rapids Basalt

Roza Basalt

Frenchman Springs Basalt

Lolo Flow: Priest Rapids Basalt

sandstone lava

lava

claystone lava claystone

claystone

lava

claystone

Potential Aquifers

Potential Aquifers

Potential Aquifer

Potential Aquifers

Hypothetical Well

Dalles Formation

Pomona Basalt

Rosalia Flow: Priest Rapids Basalt

Roza Basalt

Frenchman Springs Basalt

Lolo Flow: Priest Rapids Basalt

sandstone lava

lava

claystone lava claystone

claystone

lava

claystone

Potential Aquifers

Potential Aquifers

Potential Aquifer

Potential Aquifers

Rosalia Flow

Quincy – Squaw Creek Interbed

“Lignite”

A. Base of the Priest Rapids Member in a roadcut along I-84, just east of the Memaloose Rest Area.

B. “Lignite” beneath Rosalia (Priest Rapids Member) in Wasc 2075 (441-feet depth).

Example of a “Claystone” Interbed

Geology and

Commingling Wells

Water entering the well from a fracture in the rock (Pomona Basalt)

Water cascading down the borehole wall (near the base of the Priest Rapids Basalt).

Water moving up the space between the casing and the borehole wall.

Well Casing

Claystone

Effects of

Commingling Wells

Composite time series water level data from the Mosier area (from Burns and others, 2012)

Water Level Trends in the “Priest Rapids” aquifer W

ater

Lev

el E

leva

tion

(met

ers)

Not all wells are affected by commingling

Wat

er L

evel

Ele

vatio

n (m

eter

s)

Water Level Trends in the “Pomona” aquifer

Not all decline is caused by commingling

However, commingling does cause:

• Water level decline in many wells • Decreased streamflow in interconnected streams

Water Level Trends in the Pomona, Priest Rapids, and Frenchman Springs aquifers

Wat

er L

evel

Ele

vatio

n (m

eter

s)

Potential Equilibrium Elevations

New equilibrium elevations at interconnected streams

Summary • Knowledge of the geologic framework in the Mosier area is

key to identifying potential commingling wells.

• Some commingling wells do exist in the Mosier area.

• Commingling wells are responsible for some of the water-level declines in the Mosier area.

• Stream / Aquifer boundaries have some control on hydraulic head in the Mosier area.

• Commingling wells have contributed to decreases in stream flow.

Questions??????