Department of Geology
Eastern Washington University

Recent Masters theses in Geology

A RECONNAISSANCE OF HYDROGEOLOGY AND GROUND WATER QUALITY IN THREE HILLSIDE
BASINS AT THE PERIMETER OF THE RATHDRUM PRAIRIE AQUIFER, KOOTENAI COUNTY,
IDAHO

Deanna L. Clarkson



	From July to September, 1997, a reconnaissance investigation was
completed of the hydrogeology and ground water quality in three hillside
basins adjoining the Rathdrum Prairie aquifer in Kootenai County, Idaho.
These basins contribute recharge to the Rathdrum Prairie-Spokane Valley
aquifer system, a drinking water source protected by both state and federal
designations requiring maintenance of the high quality of its water.  The
investigation of hillside basins was implemented in response to the 1996
discovery of nitrate contamination in public drinking water supplies at the
southern margin of the Rathdrum Prairie in south Post Falls, Idaho.
Nitrate contamination is linked to septic discharge and fertilizers and
causes a potentially fatal illness in infants.  To assess the quality of
ground water in the hillside regions, three study areas were chosen to
represent basins of relatively high, intermediate, and low residential
development density as measured by the number of onsite septic systems they
contained.  These basins were:  high density, Nettleton Gulch, east of
Coeur d'Alene; intermediate density, the South Greenferry Road basin south
of the Spokane River; and low density, Hidden Valley, southwest of
Rathdrum.  Ground water samples were collected from ten existing wells in
each basin.
	To establish the quality of the ground water, the samples were
analyzed for conductivity, temperature, pH, alkalinity, total major ions
(calcium, magnesium, sodium, sulfate, chloride), total iron, and nitrite
plus nitrate as nitrogen.  Analysis of the ground water samples was
accomplished in the field and by the Coeur d'Alene branch of the Idaho
State Laboratory.  Two sampling rounds were completed:  in July, when the
water table was high, and in September, when the water table was falling.
No statistically significant differences were found between the July and
September water samples.  The measurements of the depth to the water table
in each basin established that ground water flow in the basins was toward
the Rathdrum Prairie aquifer.
	The ground water in the study basins was compared with the Rathdrum
Prairie aquifer water, as represented by the results of the July 1997
Panhandle Health District monitoring of 29 public drinking water wells on
the Rathdrum Prairie.  The ground water in the study basins and Rathdrum
Prairie aquifer presented similar calcium bicarbonate major ion profiles,
although in Hidden Valley and Nettleton Gulch, the ground water, as
evidenced by its higher conductivity, was more mineralized than the water
of the main aquifer.
	In the Rathdrum Prairie aquifer, the median nitrate concentration
was 0.971 milligrams per liter (mg/L), while the median chloride
concentration was 2.0 mg/L.  In the low density development, Hidden Valley,
the median nitrate concentration was the lowest of the study basins at
0.241 milligrams per liter (mg/L).  The median chloride concentration in
samples of Hidden Valley was 1.3 mg/L, also the lowest among the study
basins.  In Nettleton Gulch, where development density was high, the median
nitrate was 1.09 mg/L, intermediate among the study basins, while the
median chloride concentration was the highest at 4.0 mg/L.  The highest
nitrate concentrations were detected in the basin of intermediate
development, the South Greenferry Road study basin, where the median
nitrate concentration was 2.61 mg/L, and the median chloride concentration
was 3.0 mg/L.  The highest nitrate concentration of the study was 10.5 mg/L
found in a well at the site of a small farm and commercial greenhouse in
the South Greenferry Road basin.  The highest nitrate concentration in
Nettleton Gulch was 3.90 mg/L.  The elevated chloride and nitrate
concentrations in the South Greenferry Road basin and Nettleton Gulch
suggest the influence of septic discharge on the ground water.  In
addition, fertilizer was a probable source of the unexpectedly high nitrate
concentrations in the South Greenferry Road basin.  The areas of the basins
most vulnerable to ground water contamination are those with highly
permeable, low water capacity soils.
	An estimate of total nitrogen loading in the study basins suggests
that the use of fertilizers may introduce a load at least equivalent to
septic discharge.  One small farm in the South Greenferry Road basin alone
produces an annual nitrogen load equivalent to the estimated nitrogen load
from the basin's total septic discharge.
	The results of the current study imply that limiting the use of
onsite septic systems and constructing sewers is an essential part of
protecting the ground water of Kootenai County, because septic discharge is
the critical component of the nitrate load that can be addressed through
land use planning and regulation.  Control of the additional source of
nitrate contamination, fertilizer use, is dependent on the voluntary
compliance of residents educated in best management practices.