Department of Geology
Eastern Washington University

Recent Masters theses in Geology

DISTRIBUTION OF MAJOR AND TRACE METALS IN GROUNDWATER OF THE SPOKANE
AQUIFER, NORTHEASTERN WASHINGTON:  WATER QUALITY AND RIVER/AQUIFER
INTERACTION

Yi Zheng

Abstract

The Spokane aquifer consists of thick, poorly sorted glaciofluvial
sediment.  It is one of the most prolific aquifers in the United States and
serves a population of more than a quarter million people in the rapidly
growing city of Spokane and the adjoining areas located in northeastern
Washington.

This study investigates the concentration of 26 metals in the groundwater,
including 4 major elements Ca, Mg, K and Na, and 22 trace metals Al, Sb,
As, Ba, Be, Cd, Cr, Co, Cu, Fe, Pb, Mn, Mo, Ni, Se, Ag, Sr, Tl, Th, U, V
and Zn.  A total of 99 groundwater samples were collected in October 1993,
January and April 1994 and 14 river water samples in January and April
1994.  The samples were analyzed by flame AA, ICP-AES and ICP-MS following
procedures specified by the U.S. Environmental Protection Agency
Environmental Monitoring Systems Laboratory.  Historical metal data,
collected by the Spokane County Engineers' Water Quality Management
Program, were compiled to assist this study.

Hardness data indicate that wells tapping the Spokane aquifer dominantly
yield hard water.  Compared with the U.S. EPA drinking water standards,
trace metal concentrations in groundwater in the Spokane aquifer indicate
very high water quality.  In general, trace metals show very low
concentration.  The two most abundant trace metals are Sr and Ba, ranging
from 13 to 140 µg/L and 43 to 210 µg/L respectively.  Metals showing
relatively constant concentrations include Al, As, Cr, Pb, Mo, U and V.
Concentration ranges for these elements are: 1 to 8 µg/L for Al; 1 to 9
µg/L for As; 0.3 to 12 µg/L for Cr; 0.1 to 7.7 µg/L for Pb; 0.1 to 2.1 µg/L
for Mo; 0.2 to 7.1 µg/L for U; and 0.1 to 4.4 µg/L for V.  Cu, Fe and Zn
concentrations vary widely, with Cu from 1 to 47 µg/L; Fe from less than 5
up to 1945 µg/L; and Zn from 1 to 357 µg/L.  Concentrations of Sb, Cd, Co,
Mn, Ni, and Se are predominantly lower than 1 µg/L; concentrations of Ag,
Be, Th and Tl are all below 0.1 µg/L.  Some of the suggested factors
governing metal concentrations in the aquifer include:  bedrock
geochemistry, adsorption/desorption processes, pH conditions and
hydrogeological characteristics, such as groundwater flow rate.

Certain geochemical distribution patterns are recognized.  The
geographical variations of Ca, Mg, K and Na are most likely associated with
local variations in bedrock geochemistry and the influence from surface
water bodies.  The dominantly low concentrations of trace metals represent
their geochemical background in the groundwater, while some erratically
elevated concentrations of Cd, Cr, Fe, Pb, and Zn may have resulted from
sources such as plumbing materials and land use activities.  Concentrations
of As, Cr, and U indicate seasonal fluctuations, possibly due to seasonal
variations in groundwater redox conditions.

Controlling factors for metal concentrations in the Spokane River water
mainly include the bedrock geochemistry of the catchment, river/aquifer
interaction and the impact from earlier mining activities in the Coeur
d'Alene mining district in Idaho.  The Spokane River/aquifer interaction is
clearly indicated by downstream metal distribution.  During the low flow
period in January, concentrations of Ca, Mg, K, Na, Ba, Sr and U increase
dramatically at reaches with groundwater discharge to the river, indicating
geochemical contribution of groundwater to the river.  During the high flow
period, downstream variations in concentrations of these metals become
significantly reduced, indicating the diminished influence of groundwater
discharge on the river.  Seasonal fluctuations of Al, Fe, Pb and Mn
concentrations also are probably related to the influence of groundwater
discharge.  During the January low flow, groundwater discharge contributes
significantly to the river flow, which may cause substantial dilution of
the river water, resulting in lower concentrations of these metals, whereas
during the April high flow, groundwater discharge may become insignificant
in comparison with river discharge, higher concentrations of these metals
may represent their true abundance in the river.  The impact of earlier
mining activities in the Lake Coeur d'Alene area is indicated by the
consistently high concentrations of Zn in the river.