Biological and associated water-quality data for lower Olmos Creek and upper San Antonio River, San Antonio, Texas, March-October 1990

Biological and associated water-quality data were collected from lower Olmos Creek and upper San Antonio River in San Antonio, Texas, during March-October 1990, the second year of a multiyear data-collection program. The data will be used to document water-quality conditions prior to implementation of a proposal to reuse treated wastewater to irrigate city properties in Olmos Basin and Brackenridge Parks and to augment flows in the Olmos Creek/San Antonio River system.

Benthic macroinvertebrate, periphyton, and phy toplankton communities were sampled at three sites along a 4.2-kilometer reach of the Olmos Creek/San Antonio River system. Total mean densities of benthic macroinvertebrates for the three sites ranged from 1,000 to 15,000 organisms per square meter. The most abundant macroinvertebrates were the class Insecta (insects). Total densities of periphyton ranged from 700 to 86,000 cells per square millimeter. Cyanophyta (blue-green algae) and Bacillariophyta (diatoms) were the predominant periphyton organisms. Total densities of phytoplankton ranged from 940 to 21,000 cells per milliliter. Diatoms and Chlorophyta (green algae) made up most of the rest of the phytoplankton. Mean diversity index for the three sites ranged from 1.0 to 2.1 for benthic macroinvertebrates. The diversity index for all sites ranged from 1.5 to 3.1 for periphyton and 0.9 to 2.5 for phytoplankton.

Hardness ranged from 220 to 280 milligrams per liter as calcium carbonate, and alkalinity ranged from 173 to 213 milligrams per liter as calcium carbonate. The largest dissolved nitrate concentration was 1.68 milligrams per liter. Dissolved ammonia nitrogen generally was less than 0.100 milligram per liter. The largest total phosphorus concentration was 0.080 milligram per liter, over one-half of which was dissolved orthophosphate.

Total aluminum and total iron were the only trace elements in water to exceed the reporting threshold by large concentrations. Total aluminum concentrations ranged from 100 to 250 micrograms per liter, and total iron concentrations ranged from 70 to 280 micrograms per liter. Lead was the most prominent trace element in bottommaterial samples with concentrations ranging from 40 to 190 micrograms per gram. The maximum copper concentration in the bottom material was 60 micrograms per gram at site 3 (the most downstream site).

Depths and velocities, measured at sample points after benthic macroinvertebrate sampling, ranged from 0.03 to 0.30 meter and from 0.06 to 1.2 meters per second, respectively. Measurable stream discharge ranged from 0.01 to 0.27 cubic meter per second. During two of the sampling periods, no flow was at site 1.