Barton Creek Canyons Study: 1993-1995
Purpose:
The Canyons Study was initiated to compare water quality impacts to tributaries of Barton Creek from different land uses and methods of wastewater disposal in their contributing watersheds.
Overview:
Data was collected from 38 sites on tributaries to Barton Creek. Three tributaries representative of each major land use are monitored monthly for baseflow. Tributaries were categorized according to the dominant land use in their drainage area: golf course, high density residential, or rural (ranching and low density residential). Tributaries were further characterized according to the predominant method of wastewater disposal used in their drainage areas: golf courses using treated wastewater effluent for irrigation, residential areas irrigating with wastewater effluent on native land, residential areas on septic systems, residential areas on central sewage systems and rural areas with little or no commercial or residential development.
Parameters measured in the laboratory included nutrients, bacteria, and physical parameters.
Findings:
There are significant differences in baseflow nitrate, ammonia, TDS, TSS, and turbidity concentrations between watersheds draining golf courses, residential, and rural land uses.
Under most analysis groupings, golf course tributaries have higher constituent concentrations than residential tributaries, and both golf course and residential tributaries have substantially higher concentrations for these five parameters than rural tributaries.*
Baseflow data, as indicated by antecedent dry conditions, suggest that nitrate nitrogen is the most variable parameter measured in the Barton Creek watershed. A comparison of tributaries characterized by various wastewater treatment strategies reveal that golf course watersheds using sewage effluent irrigation and fully developed residential watersheds on central wastewater systems generate significantly higher nitrate concentrations in their baseflow than residential watersheds irrigating native grass areas with sewage effluent, residential neighborhoods on septic systems, or undeveloped rural watersheds.
Buffers associated with residential areas using septic systems appear to be functioning to keep excess nutrients and bacteria from reaching surface waters. This finding may also be related to the lower impervious cover associated with larger lot sizes in residential areas on septic systems.
When water samples are collected simultaneously during storm events from the three selected tributaries representing residential, golf, and rural land use, the representative golf course site is significantly higher in nitrates and ortho phosphorus, while the representative residential site is significantly higher in pH and lower in TDS. The residential site's lower TDS illustrates the effect of heavier storm runoff experienced in land uses with more impervious cover.
Baseflow water quality samples collected contemporaneously from two adjacent residential canyons on central wastewater collection systems indicate that the size of the undeveloped buffer zone around a stream may be related to water quality. Median nitrate concentrations in these two canyons indicate that water quality improves as buffer zone size increases. Furthermore, impacts to pH are mitigated by larger buffer zones.
In summary, when compared to streams representing rural land use, various parameters indicate statistically significant water quality degradation for streams representing golf or residential land use categories.
Source: City of Austin, 1997, The Barton Creek Report, Barton Creek Canyons Study: 1993 – 1995,
April 1997, http://www.ci.austin.tx.us/watershed/rptbarton.htm
* Emphasis added.
