A Reconnaissance Of Hydrogeology And Ground Water Quality In Three Hillside Basins At The Perimeter Of The Rathdrum Prairie Aquifer Kootenai County Idaho PDF Download

"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"--Document.