Contaminants in the Streambeds of San Antonio's Westside Creeks, 2014

The Alazán, Apache, Martínez and San Pedro Creeks — known locally as the Westside Creeks — are urban tributaries of the San Antonio River that flow through some of San Antonio's oldest neighbourhoods. A planned restoration of 14 miles of their channelised sections is expected to disturb the streambed, which could return chemicals held in the sediment to the ecosystem, where, depending on conditions, they could become available and toxic to aquatic life.

Urban areas like San Antonio often have high levels of contaminants in sediment. Animals that eat sediment can be harmed; trace elements and organic compounds build up in their bodies and can eventually kill them, and when they are eaten the contaminants pass up the food chain, a process called biomagnification.

The study

The U.S. Geological Survey, with the San Antonio River Authority, sampled sediment and water at sites on the Westside Creeks as a first look at contaminants there: in January 2014, at base flow, and again in May 2014, after stormwater runoff. Sediment was analysed for trace elements and organic contaminants — pesticides, brominated flame retardants, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). As a measure of ecological health, water samples were tested for toxicity to the fathead minnow (Pimephales promelas) over 7 days.

Sediment results were compared with guidelines for the animals living on the streambed: below the threshold effect concentration (TEC) harm is not expected; above the probable effect concentration (PEC) harm is expected often.

What was found

• Trace elements. Arsenic, cadmium, mercury and nickel were below the TEC in every sample. Lead and zinc most often exceeded it, and lead was above the PEC in all four samples from the upper and middle San Pedro Creek sites — possibly from old tanks that held leaded fuel or from the area's many car-related businesses.
• Pesticides. Of 19 pesticides studied, older ones banned or being phased out for their toxicity and persistence were found more often than current ones. Three chlordane compounds (cis-chlordane, trans-chlordane and trans-nonachlor) were in every sample. Pesticides most often exceeded their PECs at the upstream and middle San Pedro sites: DDD, DDE and DDT at the middle site at base flow; chlordane, DDD and DDE upstream and DDD and DDE at the middle site after the storm.
• Flame retardants. Brominated flame retardants, added to plastics, foams and fabrics in electronics, cars, clothing and furniture, were found at every site, typically above available aquatic-life guidelines (not all compounds have them). They were found more often after the storm, especially at the San Pedro Creek sites.
• PCBs, once common in plasticisers, hydraulic fluids and capacitor fluids: 18 compounds were analysed at 10 sites. PCBs 174, 187 and 194 were in every sample. Total PCBs never exceeded the TEC, but were highest at the upstream and middle San Pedro Creek sites.
• PAHs. Most individual PAHs, and total PAHs, exceeded their TECs, and PECs were exceeded in 30 of 220 analyses. At the most contaminated site, individual PAHs exceeded their PECs at base flow and after the storm, and total PAHs after the storm were 3.3 times the PEC. The PAH mix in the sediment most closely matched dust from parking lots sealed with coal-tar sealcoat, among seven published source profiles that also included car exhaust and coal-fired power plants.

Toxicity to fish

Six of the ten water samples taken at base flow were toxic to fathead minnows, markedly reducing survival and growth. Survival showed no clear link to the major ions in the water or to contaminants in the base-flow sediment. None of the ten samples taken after the storm reduced survival compared with clean control water.

Of most concern

The compounds found are common in urban streams, but the San Pedro Creek sites stood out: their sediment held lead and older pesticides that consistently exceeded PECs, and the highest total PCB levels in the study area, though below the TEC.

The fact sheet dates the second sampling "May 2104"; it was May 2014. It names the brominated flame retardants "polybrominated biphenyl ether congeners"; they are polybrominated diphenyl ethers. And it names the site with the highest PAHs as upstream Apache in one place and upstream San Pedro in another, so this page names neither.

Sources

• Cassi L. Crow, Jennifer T. Wilson and James L. Kunz, Selected Streambed Sediment Compounds and Water Toxicity Results for Westside Creeks, San Antonio, Texas, 2014, USGS Fact Sheet 2016–3096, November 2016, in cooperation with the San Antonio River Authority. https://doi.org/10.3133/fs20163096
• The full report: Crow, Wilson and Kunz, 2016, USGS Scientific Investigations Report 2016–5136.