USING SHALLOW SOIL TEMPERATURE MEASUREMENTS TO DETECT VARIATIONS IN AQUIFER TRANSMISSIVITY

Temperature measurements are valuable in ground water studies because heat flow is coupled to the movement of ground water. In particular, fluctuations in the temperature of infiltrating surface water can be used to estimate aquifer parameters. Several studies have used head and ground water temperature data to determine hydraulic conductivity. A limitation of this technique is that ground water temperature measurements are generally made in wells. In most situations, it is impractical to drill the large number of wells required to characterize a heterogeneous aquifer.

Shallow soil temperature observations are an alternative to the direct measurement of ground water temperature. Near the surface, soil temperature is controlled by atmospheric temperature, which varies diurnally. At a depth of roughly 2 m, however, the effect of this cycle is strongly attenuated, and spatial patterns due to monthly and annual temperature variations in the underlying aquifer may be detectable. Soil borings of 2 m depth can be made quickly and inexpensively. As a result, spatial sampling can be dense, with additional measurement locations easily added in zones of interest.

The USGS drilled a large number of wells near Rotterdam, NY in the 1960s to study induced infiltration from the Mohawk River. O'Brien (1970) used monthly measurements at this site to compare temperatures measured in wells with temperatures measured in 2 m deep soil borings. In both data sets, a high temperature zone can be observed in the early summer, as warm infiltrating river water travels through a high transmissivity channel to nearby water supply wells. With these data, we can both gain insight into the relationship between soil temperatures and aquifer temperatures and constrain the spatial variability of transmissivity at the field site, independently determined from pumping tests and numerical models.