CONTAMINATED GROUND WATER SITE CHARACTERIZATION USING BOREHOLE GEOPHYSICS AND A MULTILEVEL SAMPLING SYSTEM

The Cretaceous age volcanic bedrock aquifer underlying the town of Cidra, Puerto Rico, provides an important source of water for local industrial and other users. This aquifer has been impacted by volatile organic compounds from an unidentified source or sources. This paper will describe the hydrogeologic investigation methods applied by U.S. EPA under the Superfund program to characterize ground water contamination at this site and to try to identify the source.

To develop the hydrogeologic conceptual model aerial photography, geologic mapping, topographic mapping, surface water mapping, and data on potential contaminant sources were analyzed. Locations for the initial set of six monitoring wells were then chosen based on the conceptual model and the results of a soil sampling program.

Because the site is underlain by fractured volcanic rock, a suite of borehole geophysical tools were used to investigate the hydrogeology. Six boreholes were completed to depths up to 325 feet below ground surface. Caliper, natural gamma, acoustic televiewer, optical televiewer, self potential, single-point resistance, formation resistivity, fluid temperature, and fluid conductivity logs were collected in each well.

These data were used to select zones for heat pulse flow meter (HPFM) logging. HPFM logs were collected under ambient conditions and then under pumped conditions. The results of the HPFM logging were used to identify intervals for packer testing and sampling. The results of the packer testing, along with the other borehole geophysical data, were used to design a multilevel monitoring well for each location. The Flexible Underground Liner Technology (FLUTe) multilevel well system was chosen for use at the site. After the multilevel monitoring wells were installed, water levels were measured and ground water samples were collected to characterize ground water quality.

The results of the geophysical logging, water level monitoring, and groundwater sampling were used to further develop the conceptual model. Using the conceptual model, additional monitoring well locations were selected upgradient of groundwater contamination observed in the bedrock to try to identify sources in the overburden. The results of the well installation and groundwater sampling will be used to update and revise the conceptual model.