GROUNDWATER TEMPERATURE CHANGES IN A SHALLOW COASTAL AQUIFER IN RESPONSE TO CLIMATE CHANGE AND LAND DEVELOPMENT

Vertical water temperature profiles from wells in the shallow aquifer system of Virginia Beach, Virginia show anomalous warming - groundwater temperatures average 3.9 degrees C warmer than would be expected from equilibrium with deeper geothermal gradients. Local meteorological records show atmospheric warming of 1.35-1.51 degC/100 years, accounting for only about 40% of average warming observed at the top of the water column. Contrasts in groundwater temperatures between urban areas in northern Virginia Beach and rural agricultural lands in southern Virginia Beach reveal a pattern of warming recharge associated with large amounts of impervious surface cover. We present a wide variety of data to highlight the influence of land development and changes to the groundwater flow system on subsurface thermal conditions.

The northern part of Virginia Beach has experienced extensive urban development since 1940 and impervious surface cover averages 21.2%. Our results show groundwater temperatures are positively correlated with percent impervious cover and inversely correlated with percent canopy cover. Groundwater temperature correlation extends to greater depths with canopy cover than with impervious cover, perhaps reflecting that agricultural development occurred before urban development. Environmental tracer data suggest that groundwater recharge is relatively fast (<60 years) as compared to characteristic thermal diffusion times (200-500 years) for the shallow surficial aquifer. Water-use and water-level records show that seasonal groundwater use from private wells, primarily for irrigation, increases summertime downward vertical hydraulic gradients, thereby inducing and enhancing warm season recharge relative to cool season recharge. The approach demonstrated here has potential application for quantifying depths of active groundwater flow, determining spatio-temporal patterns of land use change, and estimating temperatures of groundwater discharge to sensitive wetland ecosystems.