Abstract
EPA and most States require a hydrogeologic assessment of contaminated sites to identify and characterize the upper most aquifer and potential pathways for contamination. The advancement of soil borings, followed by the completion of monitoring wells in boreholes is often the first step in direct site characterization. These monitoring wells have the potential to contaminate deeper ground waters; if the site has a contaminated seasonal perched water table from which water is allowed to enter the borehole before the well is cased. This condition is particularly troublesome when a temporary monitoring well is used for assessment purposes and contaminate concentrations are compared to maximum concentration limits (MCLs) for drinking water. A seasonal or permanent perched water table may be contaminated, but separated and protected from deeper useable groundwater by the unsaturated ‘vadose’ zone. The lack of saturation in the vadose naturally restricts the movement of water due to unsaturated flow dynamics and the corresponding flux of soluble constituents irrespective of attenuating reactions with soil. This paper presents a method of distinguishing saturated zones from unsaturated zones by comparative analysis of existing soil moisture and the field capacity or 33.3 kPa (1/3 bar) moisture equivalent. Moisture measurements are made for core samples collected in continuous fashion to a selected depth and sample interval determined by the use and sensitivity required in the interpretation of the data. Soil moisture profiles in conjunction with geophysical measurements of bulk density and particle density allow determination of volume wetness and degree of saturation. Additionally, these data are used to assess the movement of water in the soil profile and potential contaminant migration to useable groundwater. The authors have used this technique to differentiate vadose zones sandwiched between seasonal perched water and even thin continuous saturated zones from useable groundwater. Actual field data are presented to demonstrate the ease with which false positive results are generated that portends an adverse impact to groundwater and the need for costly risk reduction analysis or remediation.
