THE USE OF LEAD 207/206 ISOTOPE RATIO AND ELEMENTAL CONCENTRATIONS TO DETERMINE THE INTERACTION BETWEEN THE SPOKANE VALLEY AQUIFER AND THE SPOKANE RIVER

About 130 water samples from the Spokane Aquifer, Spokane River, Coeur d'Alene River and Little Spokane River were analyzed for the lead isotopic ratio 207Pb/206Pb and 25 elements, which include major elements Ca, Mg, Na and K and trace elements Ag, As, B, Ba, Be, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Rb, Sb, Sr, Tl, U, V and Zn.

The Spokane Aquifer naturally contains higher concentrations of Ca, Mg, Na, K, As, Ba, Cr, Mo, Rb, Sr and U and very low concentrations of Cd, Fe, Mn, Pb, Sb and Zn. The Spokane River, which is influenced by the influx of water from the Coeur d’Alene River, contains higher concentrations of Cd, Fe, Mn, Pb, Sb and Zn and lower concentrations of Ca, Mg, Na, K, As, Ba, Cr, Mo, Rb, Sr and U than the aquifer. During high and low flow in the Spokane River, concentrations of Ca, Mg, Na, K, As, Ba, Cr, Mo, Rb, Sr and U decrease in wells close to an influent reach of the Spokane River. Where the Spokane Aquifer becomes effluent, these elements increase in the Spokane River during both high and low-flow.

The 207Pb/206Pb ratio found in unimpacted wells of the Spokane Aquifer is 0.824, which is similar to ratios found in soils, sediments and unaffected surface waters of the Spokane – Coeur d'Alene area. During high-flows, the 207Pb/206Pb ratios in the Coeur d’Alene River, Spokane River and wells near the river are generally similar to the ratio found in the Coeur d’Alene mining district ore (0.945) indicating a significant impact from the Coeur d’Alene River. During low-flow, ratios decrease down stream due to discharge of aquifer water containing a ratio of 0.824.

Groundwater derived from the Spokane River can be detected in the Spokane Aquifer by using the lead 207Pb/206Pb isotopic ratio. Ratios in wells close to the Spokane River are proportional to river flow. This suggests that the Spokane River is the source of dissolved lead in wells close to the river, and the primary source is the Coeur d'Alene mining district. Significant positive correlations exist between lead isotopic ratios and major and trace elements Ca, Mg, K, Mo, Rb, Sr and U found in the Spokane Aquifer and Spokane River. These correlations, the lead isotopic ratio and the concentrations of elements mentioned above can be used to determine Spokane Aquifer / Spokane River interactions and, potentially, groundwater flow directions.