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Analysis of Pedernales River Water QualityBy Michael E. Barrett, Ph.D., P.E. ABSTRACT The LCRA water quality database for the Pedernales River was analyzed to determine current conditions and whether there are significant water quality trends. Possible trends include spatial trends, in which the concentrations vary along the length of the river, and temporal trends, meaning a variation in concentrations through time at a single monitoring site. The data were collected by the LCRA and TNRCC at four sites along the Pedernales River: RR 1320, Johnson City, RM 962, and upstream of the confluence with Falls Creek (the lower boundary of this river segment). Although the length of the monitoring period varied among the sites, the majority of the data were collected between 1984 and 1997. During this period about 7200 measurements of 24 constituents were made at the monitoring sites. A subset of these constituents was selected for detailed statistical analysis. The selected constituents include measures of oxygen content, dissolved solids, nutrients and bacteria. Analysis of the LCRA water quality database for the Pedernales River indicates that several constituents exhibit a significant spatial trend. All of the constituents that varied had higher concentrations upstream than down. These constituents are dissolved oxygen, chloride, sulfate, nitrate plus nitrite, and specific conductance (a measure of dissolved solids). The monitoring data are not sufficient for identifying the reasons for these changes, which may be natural, manmade, or a combination of the two. Temporal trends were most evident at the upstream monitoring station at RR 1320, which generally had the longest period of record. Constituents with significant temporal trends include specific conductance, chloride, and sulfate. The concentrations of each of these showed a reduction with time. The monitoring site at RM 962 showed a single parameter with a temporal trend, chloride, which also had declining concentrations. There was no significant temporal trend for any constituent at Johnson City or the Falls Creek sites. These reductions in concentration may be the result of much higher than average rainfall in 1991 and 1992. Rainfall generally has very low concentrations of these constituents, causing a dilution in groundwater, which contributes baseflow to the river, as well as in the river itself through increased surface runoff. The water quality of the Pedernales River can be characterized as very good and supportive of all designated beneficial uses. Current concentrations of all major water quality constituents are generally below levels that would be cause for concern or result in the listing of this river segment as impaired. In addition, this data set indicates that the concentrations of the analyzed constituents at each monitoring site are relatively constant or improving. There is no immediate concern that changes in land use or other human activities threaten the water quality of the Pedernales River; however, the source of higher concentrations of dissolved solids and nutrients in the upstream reach of the river should be identified. The change from undeveloped to agricultural land use occurred many years before the beginning of any water quality monitoring programs and may be responsible for higher constituent concentrations in this portion of the watershed. The highlighted links that follow are connected to Adobe pdf files of the corresponding material. To view them you must have the Adobe Acrobat Reader 3.0. View the Whole Report...(116K)
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