VARIATION IN AQUIFER HYDRAULIC GRADIENT RELATED TO NEAR-DECADAL CLIMATE CHANGES

The Eastern Snake River Plain aquifer beneath the Idaho National Engineering and Environmental Laboratory (INEEL) is a highly heterogeneous system comprised of sequences of sediment and basalt that ranges from fractured to massive. This aquifer is monitored for water level changes in several hundred wells. Hydrographs of nearly 100 aquifer wells were examined to determine correlations between recharge and water level fluctuations. Many of these well hydrographs extend for nearly 50 years and show long-term oscillations but an overall decreasing trend. The trend of decreasing water level may be attributable to long-term regional climatic trends. The long-term oscillations are of a floating frequency with a period ranging from 12-15 years. Snowpack surveys from surrounding tributary mountain ranges show cyclic behavior of similar period. We are removing seasonal and trend effects from the snowpack and aquifer water level data, using the Kolmogorov-Zurbenko (KZ) filter, to provide stationary data series for the analysis of spatial and temporal distribution of snowpack/aquifer correlations. The KZ filter is based on iterative moving averages that can remove signals of selected frequency by modifying the filter window length and number of iterations.

Calculations of aquifer hydraulic gradient, determined from the plane surface formed by the water levels in three wells, have resulted in another time-series data set. The near-decadal cycle is also seen in hydraulic gradient changes. Seasonal cycles cause about two percent or less of the gradient variation. Changes in hydraulic gradient related to near-decadal climate fluctuations appear to be five to ten percent or more. The effect on hydraulic gradient, resulting from the long-term trend of decreasing water levels, is also being determined. Past INEEL waste management practices and accidental discharges have resulted in vadose zone and aquifer contamination. Discrepancies between predicted and observed contaminant plume configurations have foiled groundwater modelers and risk assessors. Typically, these models have assumed a constant aquifer gradient. The variation in aquifer hydraulic gradient, resulting from near-decadal changes in climate, may explain many of these discrepancies.