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Carter & Burgess, Inc. has completed a water quality assessment project for the City of Abilene to determine the water quality impacts to Lake Fort Phantom Hill for a series of urban development scenarios. Lake Fort Phantom Hill is the primary drinking water supply for the City of Abilene. The land surrounding the lake consists of 438 city-owned residential lots that are serviced by septic systems. Additional developable areas were identified and simulated using the Soil and Water Assessment Tool (SWAT) model. The purpose of the project was to determine the water quality impact that would result from development around the lake including various densities of residential land use, retail/commercial property and recreational amenities. The project also examined several sewage treatment options. Alternatives to conventional wastewater collection included on-site septic systems, cluster septic systems, a small wastewater treatment plants (SBR) or a membrane bio-reactor (MBR). The SWAT model was calibrated to current conditions within the watershed as the baseline condition for water quality comparisons. Model simulations were performed for a 10-year period from 1994 through 2003 and examined pollutant loadings to the lake and resulting in-lake concentrations of nitrogen, phosphorus, sediment, chlorophyll-a, and total dissolved solids. The project also examined the water quality and yield impacts from supplemental groundwater use and municipal wastewater reuse. Six scenarios were modeled using SWAT:
1) Baseline scenario (existing conditions)
2) Baseline conditions with municipal effluent reuse
3) Baseline conditions with groundwater transfer into the lake from the Seymour Aquifer
4) Development Scenario 1 Low density residential development (2 lots per acre) and on-site septic systems
5) Development Scenario 2 Medium density (4 lots per acre) residential development with cluster septic tanks (designed under Chapter 317 rules) followed by subsurface irrigation
6) Development Scenario 3 High density (8 lots per acre) residential development with city sewer connections
This paper presents the results of the water quality simulations for the 6 scenarios and the impacts to the lake with respect to water quality and yield.