Perchlorate mystery surfaces in Texas

Researchers from Texas Tech University (TTU) have uncovered the largest area of contiguous perchlorate contamination in the United States, which exceeds 30,000 square miles in western Texas. Although concentrations are at low levels—the bulk of them in the 4 parts-per-billion (ppb) detection range—the big question now is, Where is the perchlorate coming from?

It all started when the Texas Commission on Environmental Quality (TCEQ) was conducting routine sampling under the U.S. EPA’s Unregulated Contaminant Monitoring Rule, according to Steve Walden, a TCEQ special projects manager. After detecting trace levels of perchlorate in drinking water wells supplying the Midland, Texas, area and in an elevated water storage tank in the city of Levelland, roughly 100 miles north of Midland, TCEQ continued taking samples. Surprisingly, the agency found perchlorate almost everywhere it looked.

TCEQ turned to TTU for help, and the study area was expanded to nine counties around the sites where perchlorate was found in an attempt to establish a contamination perimeter. Of the 217 public drinking wells the TTU researchers tested, 73% had detectable perchlorate concentrations of more than 0.5 ppb and 35% had concentrations equal to or above 4 ppb. California’s draft drinking water standard is currently set in a range of 2–6 ppb. The highest level found was 58.8 ppb, according to Andrew Jackson, an environmental engineer at TTU, who presented the findings in late July at a perchlorate symposium in Sacramento, Calif.

Finding no limit to the contamination, the TTU researchers expanded their study to 54 counties, an area of roughly 60,000 square miles, which is bigger than some states. “In the northern part of the Panhandle [in northern Texas], we’re finding much reduced occurrence, but in the bottom two-thirds, it’s fairly consistent,” Jackson says.

The findings could have huge implications for the state, depending on whether EPA chooses to regulate perchlorate in drinking water in the low-parts-per-billion range. EPA recommended a preliminary drinking water limit of 1 ppb in 2002 (Environ. Sci. Technol. 2002, 36, 125A), but the risk assessment on which this draft limit is based is currently under review by the National Academy of Sciences (Environ. Sci. Technol. 2003, 37, 166A–167A).

Drinking water in western Texas is typically culled from a mix of surface and groundwater sources, the latter coming from the High Plains aquifer system. Here, the underground Ogallala aquifer is the main water source, and it is where most of the perchlorate was detected. “It’s a massive water resource for the state,” Walden says, adding that regardless of EPA’s decision, people will have to continue using these water supplies because no other options exist.

Until now, most perchlorate contamination has been associated with point sources. “They usually have a tight, controlled plume produced off some industrial site that used to manufacture or handle perchlorate,” Jackson says. An occurrence survey of national drinking water supplies published in 2002 by the American Water Works Association’s Research Foundation confirms this.

In Texas, however, no plumelike pattern is discernible, according to Jackson. Moreover, perchlorate’s occurrence is random. For example, the TTU researchers have found some wells yielding relatively high perchlorate concentrations immediately adjacent to wells with little or no detectable perchlorate.

The four scenarios considered to be most plausible for generating the widespread contamination are agricultural use of fertilizers containing perchlorate, in situ generation of perchlorate by an electrochemical reaction, a natural source, or some combination of these three. For example, TTU researchers believe that the high perchlorate concentrations initially detected in Levelland’s water storage tank were generated by the unit’s cathodic protection system through mechanisms confirmed in laboratory experiments. “A lot of oil wells, pipelines, and water wells have active systems on them,” Jackson says, but the parameters necessary for perchlorate formation have only been observed with this one tank.

More likely, the patchy occurrence points to a natural mineral source, either in the subsurface water where it’s dissolved and then transported to the aquifer, in the saturated zone, or possibly even an upwelling from deeper layers, Jackson says.

Researchers have found perchlorate in the low-parts-per-billion range in some naturally occurring evaporite materials in scattered locations in the Western hemisphere, says Greta Orris, a research geologist with the U.S. Geological Survey. It’s possible that the perchlorate in Texas could be coming from such a source, “but we don’t know enough yet at this point,” she notes. Also, she says, “Because the perchlorate is frequently present in such small amounts, it’s hard to isolate enough of it to study [the source] directly and try to figure out if it’s natural or manmade.”

Orris and Jackson predict, however, that the extent of low-level perchlorate occurrence, especially in Western aquifers and likely beyond Texas, has been underestimated and is more widespread than previously thought. “Nobody was looking for it before,” Jackson says, but now that detection limits are at the sub-parts-per-billion range, “more people are going to find it.” —KRIS CHRISTEN