MEASURING GROUNDWATER STORAGE CHANGE IN AN UNCONFINED ALLUVIAL AQUIFER USING TEMPORAL GRAVITY SURVEYS

Abstract

Over 120 microGal (μGal) gravity measurements were collected at a stream augmentation site in northeastern Colorado to correlate water table fluctuations with temporal gravity data. Up to 8 m changes in water table elevation occur at this site as a result of controlled groundwater pumping and subsequent infiltration at recharge ponds. Temporal gravity data were collected in two phases: 1) During pumping of the alluvial aquifer and infiltration at the recharge ponds, and 2) after pumping had ceased and the recharge ponds were dry. Gravity data were collected with a Scintrex CG-5 Autograv meter at 250 meter spacings over a 3.2 km area, except around pumping wells where station spacing was roughly 25 meters. The difference between gravity measurements taken during each phase of the study reveals distinct gravity lows and highs at the pumping wells and recharge ponds, respectively. During phase 1, gravity was lower by as much as 106 μGal near the pumping wells and higher by as much as 98 μGal at the recharge ponds. Decreases in gravity correspond to water table drawdown at the pumping wells and increases correspond to groundwater mounding at the recharge ponds. Specific yield was estimated at seven well locations and values ranged from 0.07-0.41 with a mean specific yield of 0.23. This mean value is within the range of specific yield values (0.1-0.3) determined by aquifer tests in the area. Based on these results, the temporal gravity technique has the potential to estimate regional changes in groundwater storage and contribute critical data to water management plans.