Groundwater contamination at the Apache Powder Superfund Site in Cochise County, Arizona includes dissolved nitrate-N and perchlorate. Past studies have determined that these contaminants migrate laterally from a perched zone underlying Apache Nitrogen Products, Inc. operating areas into the shallow aquifer of the San Pedro River alluvium (shallow aquifer). The base of both the perched zone and shallow aquifer is a dense clay unit known as the St. David clay (SDC). Locally, the top of the SDC is an erosional surface with irregular topography. The perched zone occurs where artificial recharge settles in an erosional structure analogous to a “hanging valley” relative to the deeper adjacent shallow aquifer. Perched zone groundwater occurs within topographic lows. Shallow aquifer groundwater occurs under semi-confined conditions with a pressure surface at a lower elevation than the perched zone water table.
SEISMIC DATA ACQUISITION AND PROCESSING
To better define the SDC surface and the contaminant pathway from the perched zone to the shallow aquifer, and within the shallow aquifer itself, a total of four lines of high resolution, seismic reflection (HRSR) data were recorded at the site. Seismic waves were generated and data were recorded at 5-foot station spacings. Two HRSR lines (Lines 1 and 2) were recorded in the perched zone. These data were used to locate topographic lows on the clay surface for installation of new piezometers. The piezometers were intended to monitor perched zone dewatering and aid in the conceptual design of possible remedial measures. High resolution electrical resistivity (HRR) data were also recorded along each of these two HRSR lines at the same station locations. These HRR data were intended to identify potential saturated zones.
Two additional seismic lines were recorded in the shallow aquifer to refine known SDC topography on an existing groundwater monitor well transect (Line 3) and to help select drilling locations along a new groundwater monitor well transect (Line 4). The new monitor wells were needed to delineate the extent of shallow aquifer groundwater contamination and monitor groundwater quality during remediation.
DATA INTERPRETATION AND SELECTION OF DRILLING LOCATIONS
SDC HRSR data obtained along Lines 1 and 3 were “keyed” to known SDC elevations. HRSR data indicated a few topographic lows in the perched zone SDC. HRR data confirmed one known saturated topographic low and indicated two additional possibly saturated topographic lows. One low was located north of existing well P-9 on Line 1. A piezometer (MW-28) was installed at this location. Four other new piezometers (MW-29 through MW-32) were installed on Line 2 at indicated interpreted lows. HRSR data indicated that the SDC generally slopes toward the south along Line 2. HRR data indicated that saturated sediments should exist on top of the SDC at the MW-29 location.
HRSR data generally confirmed the known SDC topography along Line 3, and in particular an erosional trough that was previously interpreted by drilling data at existing monitor well MW-24. Line 4 seismic data indicated that the SDC slopes generally toward the east with localized topographic lows, although at much shallower depths than at Line 3. Three new monitor wells (MW-25 through MW-27) were installed on Line 4 at the locations of the indicated topographic lows.
Overall, the seismic data served their intended purpose and were useful in the selection of new drilling locations in the perched zone and shallow aquifer. Perched zone HRSR SDC depths were within 2 feet of actual SDC depths at four drilling locations. At one location, the seismic depth was approximately 9 feet shallower than the actual SDC depth. The topographic low north of well P-9 (location of MW-28) was confirmed but was unsaturated. The seismic SDC topographic slope toward the south along Line 2 was confirmed by actual SDC data. One location indicated by HRR data to be saturated (MW-29) was confirmed by actual site conditions during drilling.
Shallow aquifer HRSR data collected along Line 4 however, indicated that the SDC depth was approximately 90 feet shallower than actual conditions encountered during drilling. The general eastward seismic SDC slope, however, was confirmed. The shallower HRSR SDC surface was probably interpreted as a result of a weakly-cemented SDC-like material (silty clay) deposited directly on top of the SDC, instead of the typical shallow aquifer materials (sands/gravels) found at other locations (Line 3). The weakly-cemented silty clay could have a similar seismic velocity to that of the SDC and may result in a higher indicated SDC elevation.
The combination of geophysical exploration and intrusive geologic/hydrogeologic investigations can be a powerful and cost-effective investigative tool in contaminant hydrogeology. The siting of additional exploratory borings or monitor wells at a site where significant subsurface geologic data are already available is typically based on various assumptions. Such assumptions are often based on the generalized trend(s) of geologic strata and hydrogeologic concepts. Responsible investigators strive to maximize the information content derived from the available funds for the study. Thus, confirming assumption(s) through the application of non-intrusive geophysical methods prior to installing new wells can be cost effective. Iterative collection of seismic data and boring or well construction provides a potentially more effective approach.