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2011-02-03

BCRAGD buys geophysical logging equipment

By Carolyn B. Edwards BCC Staff Writer

At their quarterly meeting on Jan. 6, the directors of the Bandera Country River Authority and Groundwater District (BCRAGD) approved an interlocal agreement with three other area water districts to purchase geophysical logging equipment.
The equipment has been ordered from a Denver company and should be on its way. The district will join with Hill Country Underground Water Conservation District, Central Texas Groundwater Conservation District and Blanco-Pedernales Groundwater Conservation District for the shared purchase, use, operation and maintenance of a geophysical logging trailer, logging equipment, logging tools, generator and related equipment. Water board directors had previously approved the purchase. BCRAGD's share of the cost will run to $15,000.
Explaining one of the reasons for obtaining the equipment, BCRAGD Assistant Director David Mauck noted, "By logging a well, we'll be able to determine the thickness of the aquifer and where it is."
Most of Bandera County's underground water comes from the Trinity Aquifer, a family of aquifers running west of the Balcones Escarpment from Dallas-Fort Worth to San Antonio. Unlike the better-known Edwards Aquifer, a huge underground pool that provides water for San Antonio, the Trinity is broken up into smaller pools.
Those subdivisions of the Trinity include the Glen Rose, Cow Creek and the Sligo Hosston. Wells may have to go down to 1400 feet in the Medina Lake area to hit the generally good water in Cow Creek. Elsewhere in the county, Cow Creek wells are only 500 feet deep. The Sligo Hosston provides water to the City of Bandera's wells, but the apparent quality of the water in that level deteriorates in the western reaches of the county.
"Unlike the Edwards, we can't just stick a dipstick into it and say we're at so many feet today," said Mauck.
General Manager David Jeffery expects the logging equipment to give the water district plenty of good hard data over the next few years about the Trinity that lies below the county.
"The equipment will be mounted on a trailer with a generator," explained Jeffery, which will make it easily portable both within the local water district and the three other districts. The four districts will rotate use of the logging equipment throughout the year.
"When a driller has completed a well," said Jeffery, "there is about a two-hour window of opportunity that we will have to run the log before the driller wants to put in the casing."
The equipment will include a drum wound with 1,500 feet of steel cable with an assortment of attached measuring tools. The measurements are recorded on a monitor, which prints out an electric log and stores the report on a computer.
The three basic tools included in the purchase are a gamma ray tool, a caliper tool and a resistivity tool. Gamma rays can determine the difference between clays, limestone, gypsum and other mineral layers below ground. The caliper will measure an open hole, recording variations in the hole, including caves. The resistivity tool will indicate the conductivity of the water, which reveals the relative quality of the water.
"We have been using a company called Geo-Cam to log some wells," said Jeffery, "but they have become so busy that it is hard to get them to come out when a well is available."
Other tools Jeffery wishes the district could have afforded with this purchase include a camera that would run to 1,500 feet and a tool that measures porosity. "We have a camera that will go to 500 feet," Jeffery said, "but both of these things would have added significantly to the cost."
Training is included in the cost of the equipment and Jeffery expects that at least two people from each cooperating district will receive the training both to learn to operate the equipment and to decipher the logs. Training should be held in February or early March.
Jeffery will use the information gleaned from the logs to determine the depths of the various levels of the Trinity and the apparent water quality. "We can use this information to advise water well drillers and prospective land purchasers. As we get more data, we can plug it back into the models of water usage we are developing."
Water well drillers also make a log when drilling a well, but their log information is a simple report of what the driller saw at various depths of the well as it was drilled. "It's not very specific," explained Jeffery, "but it will be useful to compare the drillers' logs with what we learn from the new logging equipment."