Upper San Marcos River Watershed Annual Water Quality Data Summary and Analysis Report FY20




Arismendez, Sandra

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In September 2018, the nine-element Upper San Marcos River Watershed Protection Plan (USMR WPP) was approved by US Environmental Protection Agency and Texas Commission on Environmental Quality (TCEQ). The USMR WPP sought to reduce water resource concerns by addressing water quality and quantity issues in the Upper San Marcos River Watershed. The plan addresses the 2010 TCEQ CWA§303(d) listed impairment for total dissolved solids (TDS), and additional parameters including E. coli, nutrients, sediment, and other pollutants associated with growth and development in the watershed. The TCEQ, through its Texas Water Quality Standards, designates appropriate uses for the state’s surface waters including aquatic life, recreation, and sources of public water supply. The criteria for evaluating support of those uses include dissolved oxygen, temperature, pH, TDS, toxic substances, and bacteria. The USMR consistently has better water quality than the state’s water quality standards and screening levels. For this reason and because of the river’s unique groundwater driven system, stakeholders developed water quality goals that were more stringent than the state’s standards as part of the USMR WPP (Table 1). Adverse effects on water quality resulting from urbanization and development within the USMR Watershed have been observed. Changes in water quality in the watershed are likely due to seasonal and annual climate variability, nonpoint source pollution (NPS), recharge and spring flow, and changes in land use and/or management. Issues of concern include excess sediment, high bacteria concentrations, and occasionally very high nutrient levels. Nutrient levels indicate potential NPS’s of pollution including pet and animal waste, excess fertilizer application, and poorly performing septic systems. As more people move to the area, the watershed becomes burdened with increased stormwater runoff and pollutant loadings. TCEQ designated the Upper San Marcos River (Segment 1814) a classified freshwater stream extending from a point 1.0 km (0.6 mi) upstream of the confluence of the Blanco River in Hays County to a point 0.7 km (0.4 mi) upstream of Loop 82 in San Marcos in Hays County including Spring Lake. The segment is in the Guadalupe River Basin in the central Texas hill country and the Upper San Marcos River watershed (Figure 1). This report is being prepared to fulfill Task 6.3 Annual Water Quality Data Summary and Analysis Report for the Upper San Marcos River Watershed Protection Plan (WPP) Implementation TCEQ contract (#582-18-80176). The purpose of this task deliverable is to analyze surface, storm, and ground water quality data collected by watershed partners to determine changes and trends in water quality over time as the management measures described in the WPP are implemented.



San Marcos River Watershed, water quality, conservation


Arismendez, S. (2020). Upper San Marcos River Watershed annual water quality data summary and analysis report FY20 (Report No. 2020-03). Texas State University, San Marcos, Texas.


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