There are additional aquifers with similar genesis and lithology to the Edwards (Balcones Fault Zone) Aquifer. Due to a multitude of commonalities, these aquifers are treated in a manner similar to the Edwards Aquifer because society relies on these resources and these aquifers face the same threats. The associated aquifers include the Edwards-Trinity (Plateau) and the Washita Prairies aquifers (Figure 2 and Table 1). The following subsections provide brief descriptions of these aquifers.

4.3.1 Edwards-Trinity (Plateau) Aquifer

The Edwards-Trinity (Plateau) Aquifer is a karstic carbonate aquifer that extends across the uplifted Edwards Plateau in west-central Texas. The plateau west of the Pecos River is referred to as the Stockton Plateau. Key studies of the Edwards-Trinity Aquifer have been conducted by the U.S. Geologic Survey (Kuniansky, 1989; Kuniansky and Holligan,1994; Barker et al., 1994; Barker and Ardis, 1996), the Texas Water Development Board (Walker, 1979; Rees and Buckner, 1980; Mace et al., 2000; Jones and Anaya, 2009, 2019; Mace and Angle, 2004; Jones et al., 2011; Hutchison et al., 2011), and Green and Bertetti (2010). The combined groundwater flow system of the Edwards-Trinity (Plateau) Aquifer and the Hill Country portion of the Trinity Aquifer covers about 100,000 km² (39,000 square miles). The aquifers are the primary source of water for the Edwards Plateau and the Texas Hill Country and sustain numerous springs and streams in the region. The sensitivity of the aquifers to drought and well discharge has raised concerns over the availability of water from these aquifers.

Studies of the Edwards-Trinity (Plateau) Aquifer and Hill Country portion of the Trinity aquifers indicate that groundwater discharge takes the form of:

• flow to streams and springs;
• evapotranspiration;
• pumpage from wells; and,
• cross-formational flow through the Balcones Fault Zone boundary to the Edwards (Balcones Fault Zone) Aquifer and underlying parts of the Trinity Aquifer.

Recharge to these aquifers occurs by diffuse and discrete infiltration through the aquifer outcrops.

4.3.2 Washita Prairies

The Washita Prairies Aquifer (Figure 2) is a shallow, unconfined aquifer that provides water to small perennial springs, rural households, and livestock (Cannata, 1988; Collins, 1989; Myearick, 1989; Bernhardt, 1991; Legg, 1995; Clark, 2000b; and Yelderman, 1987, 2019). This aquifer is relatively thin (< 50 m). The Georgetown and Edwards formations form the aquifer similar to the Northern segment of the Edwards Aquifer. Permeability is less than other Edwards aquifers discussed above. Because the aquifer is west of the Balcones Fault Zone, there is less fracturing and less karstification. Weathering created higher effective porosity and permeability in the uppermost aquifer strata. Fracture permeability controls the flow system. The water-table elevation mimics surface topography and responds rapidly to precipitation. Most discharge occurs in local second-order streams.