MEASURING GROUNDWATER STORAGE CHANGE IN AN UNCONFINED AQUIFER WITH HIGH-PRECISION TEMPORAL GRAVITY SURVEYS

Estimates of groundwater availability and aquifer lifetime at a regional scale are based in part on relatively few measurements of specific yield. In places, water is depleted at a faster rate than predicted using values based on these sparse data. The refinement of specific yield is done by a combination of additional measurements and flow-model calibration to temporal water level data. However, calibration points are often separated by 10's of kilometers. To improve calibration, a method to rapidly determine temporal and spatial changes in groundwater storage between points is needed. One method meeting these criteria is the microGal gravity technique, a surface geophysical method that measures the minor changes in gravity resulting from changes in groundwater mass.

Two microGal gravity surveys were conducted at a stream augmentation site in northeastern Colorado. Here, water is pumped from an alluvial aquifer into upland recharge ponds in eolian sands, and returns to the river via subsurface flow. Changes in water table elevation of up to 8 m around the pumping wells and recharge ponds have been observed, making this an excellent site to test the microGal gravity technique. Gravity data were collected at over 140 stations with a Scintrex CG-5 Autograv meter in two phases: the first survey was performed when the augmentation system had been in operation for 4 weeks; and the second survey was done 6 weeks after the pumps had been shut off. Station spacing was roughly 250 meters over an area of 3.2 km2, and at roughly 25 meters around the pumping wells. Comparison of the two gravity data sets shows relative gravity decreases near the pumping wells and relative gravity increases near the recharge ponds. The specific yield of the aquifer was calculated by dividing the change in water mass estimated from the gravity data by the change in water level measured in nearby wells. The mean value of specific yield determined from the gravity data is 0.20, which is the value determined by other methods.

The success of using high-precision temporal gravity surveys at this site to measure changes in groundwater storage shows that this technique has the potential to quantify changes in groundwater storage at a regional scale. This information will be critical for estimating the lifetime of aquifers and designing water management plans.