Mapping the Aquifers Beneath Camp Stanley, Texas, in 3D

By Charles D. Blome and Allan K. Clark, U.S. Geological Survey. Fact Sheet 2017–3090, February 2018, prepared with Camp Stanley Storage Activity and Parsons Corporation.

The Camp Stanley 3D EarthVision™ model: land surface, the rock units beneath, and faults (black lines). Pink Edwards aquifer rock caps some hills. View from the east. USGS.

Why build a model

To predict how much water an aquifer holds and where it moves, you need its geologic framework. Mapping the rock units at the surface shows where the permeable zones are; a 3D model shows how they connect across faults and fractures, and which layers are saturated. Its data can also feed groundwater flow models such as MODFLOW.

Over 14 years, USGS projects funded by the National Cooperative Geologic Mapping Program worked out the geology of the Edwards and Trinity aquifers of central Texas and the Arbuckle-Simpson aquifer of Oklahoma, with new hydrostratigraphic mapping, 3D models, airborne geophysics and ways to link geologic and flow models. One place where much of this came together: the U.S. Army Camp Stanley Storage Activity (CSSA), in northwestern Bexar County, about 19 miles northwest of downtown San Antonio.

Figure 1: the study area in Bexar County, and the region's major aquifers. USGS, after Pantea and others (2014).

The base and its contamination

• Size: about 4,000 acres, beside the much larger Camp Bullis to the east and southeast.
• 1906–1907: the government bought six tracts — the Leon Springs Military Reservation. Named Camp Stanley in October 1917.
• Since 1933: an ammunition depot. Its mission is still to receive, store, issue and maintain ordnance, and to test and maintain weapons and ammunition.

A century of operations contaminated soil and groundwater, on and off the base. In 1991 the Texas Department of Health found the dissolved cleaning solvents PCE, TCE and cis-1,2-DCE above the maximum contaminant levels; other volatile organic compounds turned up in a well near the base's center, and nearby wells showed even higher levels.

• Plume 1: traced to a former oxidation pond.
• Plume 2: in the base's southwestern corner.

Starting in 1996, 122 monitoring wells were put in.

The aquifer at risk

The solvents reached the Middle Trinity aquifer — the main water source for the base and the area around it. It is made up of:

• the lower member of the Glen Rose Limestone;
• the Bexar Shale and Cow Creek Limestone members of the Pearsall Formation.

The lower Glen Rose was first split into six informal units, A–F (Blome and Clark, 2014), later named the Bulverde, Little Blanco, Twin Sisters, Doeppenschmidt, Rust and Honey Creek units (Clark and Morris, 2015). Locally, the Bexar Shale is the confining layer between the water-bearing Glen Rose and the Cow Creek.

Figure 3: how the mapped rock units line up with the hydrostratigraphic and model units. USGS, after Pantea and others (2014).

The work

In 2009, talks with the base's environmental program manager led the USGS to meet with the CSSA Environmental Office, the EPA, Parsons and others. The top need: to understand the local geology and which way contaminants flow — on the base and in residential areas to the south and west. The USGS then agreed three objectives with the base, for two years.

1. A 3D model

The EarthVision™ model was built from surface maps and borehole geophysics. Changes along the way nearly doubled its area and added new field mapping to place every modeled fault and check every well. Finished in 2013, it holds 11 hydrostratigraphic units, from the Edwards aquifer through the Upper and Middle Trinity; the top of the Hammett Shale is the lowest, used to project and check faults and drill-hole data.

From the air: the model also uses a helicopter electromagnetic (HEM) survey of Camp Stanley and Camp Bullis, flown in December 2003, which maps electrical resistivity underground. It sharpened the location of known faults and suggested many more unmapped ones.

Figure 5: the HEM survey at 115 kHz. Reds: resistive rock such as limestone; blues: mudstone and shale. Black lines: base boundaries; wavy blue lines: main drainages. USGS.

The geology: Lower Cretaceous shale, sandstone, carbonate and evaporite rock of the Trinity Group, laid down on a vast, shallow marine carbonate platform (the Comanche shelf), under the Edwards Group's carbonate, chert and evaporites, from open-sea to tidal-flat settings. The model was built on the geologic map of northern Bexar County (Clark and others, 2009; updated 2016). Only the lower part of the Edwards aquifer is present, capping hilltops south of the base — it is not a recharge area here.

Figure 6: geologic map of the model area, with faults and wells. Red: the base; blue-green: the 3D model. USGS (Clark and others, 2016).

Faults: part of the Miocene Balcones fault zone, running southwest to northeast — normal faults in en echelon steps, downthrown to the southeast. The model revealed unknown horsts and grabens; one graben in the northeast, trending northeast–southwest, may steer groundwater.

• Dips: 52 to 75 degrees.
• Offsets: mostly 6 meters or less; larger on a set of closely spaced faults near the base's southern edge, and up to 60 meters on one fault in the south-central area.

Figure 8: the faulted top of the Hammett Shale, with a cross section through every model unit. View to the north. USGS (Pantea and others, 2014).

2. Logging wells

In 2012 the USGS logged 12 wells — 9 on the base, 3 off it — including MW9-CC to more than 470 feet and MW5-LGR to more than 450. Four were logged with a neutron tool, which measures the hydrogen in the rock and so its effective porosity.

3. Measuring the rock

Cores from MW9-CC and MW5-LGR went to the USGS Core Research Center in Denver to be stored, slabbed and described. 55 core plugs from the lower Glen Rose in MW5-LGR went to Weatherford Laboratories in Golden, Colorado, for helium-gas injection tests — a standard method, and the one preferred for poorly consolidated samples.

These results and the neutron logs will feed 3D property models of the area. The work was paid for largely by the National Cooperative Geologic Mapping Program and the CSSA Environmental Program.

Sources

Based on Characterizing the Subsurface Geology in and Around the U.S. Army Camp Stanley Storage Activity, South-Central Texas, by Charles D. Blome and Allan K. Clark, USGS Fact Sheet 2017–3090, U.S. Geological Survey; a work of the United States government in the public domain. The import had scrambled the fact sheet's text and left out its figures; the order and six figures are restored from its PDF. Its Figure 2, a map of the plumes by Parsons used with permission, is left out, as is Figure 7. The fact sheet names PCE and TCE "tetrachloroethane" and "trichloroethane" in its text but "tetrachloroethylene" in its figure; it gives the model's layers as 1 + 2 + 9, which is 12, beside a total of 11; and it spells the shelf "Commanche."