Kinney Pool: Defining the western boundary of the Edwards (Balcones Fault Zone) Aquifer, Texas Open Access

The Edwards (Balcones Fault Zone) Aquifer is typically defined as having three segments from west to east, the San Antonio segment, the Barton Springs segment, and the Northern segment (see Sharp et al., this volume, Chapter 1, figs. 2 and 3 therein). The segments are separated by groundwater divides or points of discharge. The San Antonio segment of the Edwards Aquifer has been further delineated into two pools, the San Antonio Pool and the Uvalde Pool, for water management purposes (Texas Senate Bill 1477, 1993). The western portion of the San Antonio segment has not been investigated as extensively as other portions of the Edwards Aquifer, and, as a result, the western boundary has not been as well characterized. Recent evaluations of the water resources of Kinney and Uvalde Counties, Texas, provide the basis to refine the characterization of the western end of the San Antonio segment of the Edwards Aquifer. Information on geologic structure, water chemistry, and hydrogeology has been integrated to develop an updated conceptual model of the Edwards Aquifer in Kinney and Uvalde Counties (Green et al., 2006; Fratesi et al., 2015). A reinterpretation of the western boundary of the San Antonio segment is formulated where the Edwards Aquifer in Kinney County is characterized as a separate pool. This pool, the Kinney Pool, extends from the groundwater divide between Pinto Spring and Mud Spring in the west to eastern Kinney County in the east (Fig. 1). The previous conceptualization that groundwater flowed from Kinney County east to Uvalde County is revised to interpret groundwater flow as northeast to southwest in the Kinney Pool.

Historical conceptual models of the Edwards (Balcones Fault Zone) Aquifer in Kinney and Uvalde Counties, Texas, were based on studies by Sayre (1936), Sayre and Bennett (1942), Bennett and Sayre (1962), and Welder and Reeves (1962). Sayre and Bennett (1942) designated a groundwater high in western Kinney County as the boundary between the Edwards-Trinity Aquifer in the west and the Edwards Aquifer in the east. The Edwards Aquifer was considered a contiguous aquifer to the east of this divide. Groundwater flow was interpreted to be from west to east from Kinney County to Uvalde County, mostly predicated on groundwater elevations that decreased monotonically eastward from the groundwater divide in western Kinney County. Subsequent studies also placed the groundwater divide and western boundary of the Edwards Aquifer near Brackettville (Bush et al., 1992; Hovorka et al., 1993; LBG-Guyton Associates, 1995; Maclay, 1995; Groschen, 1996; Khorzad, 2003; Snyder, 2004), although the specific location of the groundwater divide varied somewhat in the different studies. Similarly, most numerical models of the San Antonio segment of the Edwards Aquifer prescribed a groundwater divide in central Kinney County as the western boundary (Klemt et al., 1979; Maclay and Land, 1988; Thorkildsen and McElhaney, 1992; Lindgren et al., 2004), with the possible exception of regional Edwards-Trinity Aquifer models by Kuniansky and Holligan (1994), in which the Brackettville groundwater divide was hardly perceivable. In general, the groundwater divide near Brackettville had been historically accepted as the western boundary of the San Antonio segment of the Edwards Aquifer (Lindgren et al., 2004; Edwards Aquifer Authority, 2009).

Bennett and Sayre (1962) made an important observation that some groundwater in central Kinney County is not flowing at right angles to groundwater elevation contours, a feature indicative of anisotropic or preferential flow (i.e., karst development and karst flow or fault/fracture anisotropy). Bennett and Sayre (1962) inferred from prompt increases in discharge at Las Moras Springs in response to rainfall in the Edwards Aquifer recharge zone that well-developed solution channels extend from the outcrop area to the springs in the south. They suggested similar solution channels exist north of Pinto and Mud Springs. Bennett and Sayre (1962) identified synclines in the geologic structure that plunge to the southwest (S50°W). They noted that these structures could complement the effects of solution channels to funnel water from the recharge and contributing zones toward Las Moras Springs. To support their conceptual model that significant flow is to the east from Kinney County to Uvalde County, Bennett and Sayre (1962) further surmised there are E-W–trending solution channels. Although most of this conceptualization has been shown to be valid, no supporting evidence of this last feature purporting west-to-east flow has been found.

Additional data were collected and interpretations were refined to provide an expanded basis for interpretation of the western boundary of the Edwards Aquifer (Green et al., 2006). This included development of a detailed piezometric map, refinement of the structural geological framework of the study area, compilation and interpretation of all relevant water chemistry data, and a refined interpretation using the integrated data.

A piezometric map based on a synoptic groundwater elevation survey conducted in Kinney and Uvalde Counties in January–February 2006 is illustrated in Figure 2. In general, the piezometric surface illustrated by this map is consistent with the map by Maclay and Land (1988), with groundwater elevations highest in northern Kinney County and lowest in eastern Uvalde County. The contour map of the piezometric surface for January–February 2006 (Fig. 2) is interpreted to indicate a groundwater divide located to the west of Brackettville (Green et al., 2006). There are limited groundwater elevation data in the area to the west of Brackettville with which to define the precise location of the groundwater divide, but the divide is interpreted to be between Mud Creek on the west and Pinto Creek on the east, approximately 15 km west of Brackettville.

Structural interpretations used to develop the geological framework included those of Holt (1959), Barnes (1977, 1983), Collins and Hovorka (1997), Clark and Small (1997), and Clark (2003). Well and map data were used to construct surfaces of the top and base of the Edwards Aquifer (Holt, 1959; Welder and Reeves, 1962; Collins and Hovorka, 1997; Texas Water Development Board, 2018; Halihan et al., 2000; Ewing, 2005; Uvalde County Underground Water Conservation District, 2005). A contour map of the saturated thickness of the Salmon Peak Formation of the Edwards Aquifer in Kinney County and western Uvalde County is illustrated in Figure 3. A vertical profile of the geology from northwest Kinney County to southwest Uvalde County is illustrated in Figure 4.

The Edwards Aquifer in Kinney County is located in the Maverick Basin facies, which is subdivided into the Salmon Peak, McKnight, and West Nueces Formations. Of these formations, only the Salmon Peak can be reasonably considered an aquifer. Few wells have been developed in either the shale-rich McKnight or West Nueces Formations due to their limited transmissivities. The saturated thickness of the Salmon Peak Formation represents that portion of the Edwards Aquifer in Kinney County that functions as a water-bearing unit (Fig. 4). Areas of the map showing no saturated thickness in the Salmon Peak Formation of the Edwards Aquifer indicate that the aquifer is dewatered. Note that flow rates in the bad-water zone of the Edwards Aquifer are less than in the freshwater zone and that there is limited exchange between the freshwater and bad-water (saline-water) zones (Harden, 1968; Groschen and Buszka, 1997; Sharp and Smith, this volume; Fig. 2). The bad-water line (freshwater/saline-water interface) defines the downdip extent of the Kinney Pool. This line is sufficiently close to the southern extent of the Kinney salient to restrict west-to-east fresh groundwater flow.

The geologic cross section in Figure 4 includes groundwater elevations from the 2006 synoptic survey relative to the structure of these units (Green et al., 2006). A factor of prime importance is that the superposition of the geologic structure and the groundwater elevations indicates a virtual dewatering of the 100-m-thick permeable Salmon Peak Formation of the Edwards Aquifer slightly west of the Kinney and Uvalde County lines (Texas Water Development Board, 2018). A salient in eastern Kinney County is the principal reason the Salmon Peak Formation is unsaturated. As a consequence of this geologic structural barrier, there is limited opportunity for groundwater to flow from west-to-east from Kinney to Uvalde County, even though the groundwater elevation in Kinney County is greater than the groundwater elevation in Uvalde County. This dewatering of the Edwards Aquifer in eastern Kinney County forms a structural, hydraulic barrier that defines the eastern boundary of the Kinney Pool and the western boundary of the Uvalde Pool of the Edwards Aquifer.

Water chemistry data were assessed to identify the chemical characteristics of Edwards Aquifer waters in Kinney and Uvalde Counties. Historical data on water chemistry are from numerous sources (Texas Water Development Board, 2018; strontium data collected by the Edwards Underground Water District, John Waugh, 2005, personal commun.; Livingston, 1947; Holt, 1959; Petitt and George, 1956; Bennett and Sayre, 1962; Welder and Reeves, 1962; Pearson and Rettman, 1976; Maclay et al., 1980; Groschen, 1996; Edwards Aquifer Authority hydrological status reports [Hamilton et al. 2004]). Magnesium data for Edwards Aquifer wells show differences in the chemistry of Edwards Aquifer waters in Kinney and Uvalde Counties and generally indicate the location of the freshwater to saline-water transition south of U.S. Highway 90 (Fig. 5). The differences in magnesium concentrations and concentrations of other constituents (e.g., sulfate) of waters in Kinney and Uvalde Counties support an interpretation of two separate pools. Rocks of the Edwards Aquifer undergo a facies transition from the Maverick Basin to the Devils River Trend west to east across Uvalde County, so it might be argued that the facies transition produces the differences in water chemistry. However, the location of the transition has been well defined (e.g., Clark and Small, 1997; Clark, 2003) to occur east of the city of Uvalde and thus does not provide an explanation for the observed groundwater quality differences.

The Edwards Aquifer in Kinney County can be characterized as two NE-SW–oriented embayments (see Green et al., this volume, Chapter 5; Fig. 5). The western embayment is coincident with Pinto Valley and is referred to as the Pinto Valley embayment. The eastern embayment trends from Grass Valley in the northeast to Las Moras Creek in the southwest and is referred to as the Grass Valley embayment. The eastern boundary of the Pinto Valley embayment is punctuated by a structural high in the base of the Edwards Aquifer, referred to here as the Kinney salient. Insufficient structural evidence is available to identify the embayments as synclines or grabens. The interpretation that the embayments are synclines is consistent with the interpretation by Bennett and Sayre (1962). Conversely, faulting mapped in Kinney County suggests that the embayments are grabens (Green et al., 2006). Regardless of their genesis, the greater depths in the base of the Salmon Peak Formation in the embayments explain, in part, the presence of high-capacity wells in Pinto and Grass Valleys and low-capacity wells elsewhere.

Tracer tests were conducted to determine direction of groundwater flow in the Edwards Aquifer in Kinney County. There were two phases in the tracer testing, one performed in 2007 and the second performed in 2009 (Johnson and Schindel, 2015). Dyes used for tracing included tinopal CBS-X, rhodomine, uranine, eosin, and sulforhodamine B. Injection locations and the inferred direct pathways from points of injection to points of detection are graphically illustrated in Figure 6. As expected, tracer detections were associated with locations of wells with low Mg concentration, which is consistent with the conceptualization that occurrence of low Mg helps to define the Kinney Pool. Groundwater flow in the Kinney Pool remains in the Kinney Pool. Dye tracer testing was conducted during periods of relatively low recharge, and so groundwater travel times were relatively long. There are few monitoring points near the Kinney-Uvalde County line area because of the absence of viable groundwater wells in the area where the Salmon Peak Formation is dewatered. Regardless, there is no evidence of flow from the Kinney Pool to the Uvalde Pool. In fact, a majority of detections of tracers to the east in Kinney County were correlated with outcrops of Edwards Aquifer rocks along incised drainage features. These dye tracer results are consistent the conceptualization that groundwater in the Kinney Pool flows toward Las Moras and Pinto Springs in south-central Kinney County and not from Kinney to Uvalde County.

Groundwater flow in the Edwards Aquifer from Kinney to Uvalde County is stage dependent. No flow would be expected during periods of low to moderate groundwater elevation, but limited flow from west to east is possible during periods when groundwater elevations are high. This limited hydraulic communication is interpreted as justification to designate this part of the Edwards Aquifer in Kinney County as a separate pool (Green et al., 2006).

Potential sources of recharge to the Edwards Aquifer in the Kinney Pool are: (1) surface recharge focused in streambeds, (2) distributed recharge (also referred to as diffuse recharge) from precipitation in interstream areas, and (3) subsurface recharge from the Trinity Aquifer where the Trinity Aquifer is fault-juxtaposed with the Edwards Aquifer or where there is sufficient hydraulic communication between the two aquifers through an alternative mechanism such as preferential flow along fault surfaces. Of these potential sources of recharge, subsurface recharge is the most difficult to quantify. Typically, the only meaningful way to estimate this quantity is to determine all other sources of recharge, measure the total discharge, perform a water-balance analysis, and back calculate the amount of recharge in the subsurface.

The area that contributes to recharge of the Kinney Pool is not well defined. The contributing area has typically been equated to the West Nueces River watershed (Fig. 1; see Green et al., this volume, Chapter 4; Bennett and Sayre, 1962; Mace et al., 2004). This interpretation is predicated on the assumption that surface watersheds coincide with groundwater basins. However, the northern extent of the Kinney Pool groundwater basin is not well defined. It is possible that portions of the upper reaches of the West Nueces River watershed also recharge the Uvalde Pool. Geochemical analyses of the Edwards-Trinity Aquifer systems corroborate the conceptualization that different reaches in the West Nueces River watershed provide recharge to both the Kinney and Uvalde Pools; however, the data are not conclusive (Nance, 2004).

An observation by Bush et al. (1992), stating that groundwater flow in the Nueces–West Nueces River watersheds is to the southwest, supports the conceptual model of a separate Edwards Aquifer pool in Kinney County. This is an example of subterranean groundwater piracy, in which the flow of surface water is to the southeast in the West Nueces River watershed, and the flow of groundwater is to the southwest in the in the northeast portion of the Kinney Pool. Subterranean piracy in karst terrains is not uncommon, particularly in karst limestone aquifers such as the Edwards Aquifer (Ford and Williams, 1989; White and White, 2001; White, 2006; Green and Bertetti, 2010). Extending the observation by Bush et al. (1992), the conceptual model for a separate Kinney Pool indicates that recharge from the middle reaches of the West Nueces River goes to the Kinney Pool, and recharge from the lower reaches of the West Nueces River goes to the Uvalde Pool. This transition from the middle reach to the lower reach of the West Nueces River is interpreted to be at the eastern boundary of the Kinney Pool. If this interpretation is valid, the ultimate destination of groundwater discharged from the Kinney Pool of the Edwards Aquifer is the Rio Grande Basin, not the Nueces River Basin.

Recharge to the Edwards Aquifer in Kinney County was first estimated by Bennett and Sayre (1962) at 86 × 10⁶ m³/yr (70,000 acre-ft/yr) for the period 1939–1950, which did not include the drought of record during the 1950s. Robert S. Kier Consulting (1998) also estimated Kinney County recharge at 86 × 10⁶ m³/yr (70,000 acre-ft/yr), factoring in contributions from Real and Edwards Counties and assuming an average recharge rate of 3.5 cm/yr. Robert S. Kier Consulting (1998) estimated recharge would be 111 × 10⁶ to 117 × 10⁶ m³/yr (90,000 to 95,000 acre-ft/yr) if the average distributed recharge rate is 5 cm/yr. Khorzad (2003) modified the basin size calculated by Bennett and Sayre (1962) to arrive at 88 × 10⁶ m³/yr (71,382 acre-ft/yr) recharge from the Nueces River–West Nueces River basin. The analysis by Mace et al. (2004) provides the most comprehensive estimate of total recharge to the Edwards Aquifer in Kinney County. The Mace et al. (2004) analysis corrected the errors in the watershed area and streamflow cited in the Bennett and Sayre (1962) analysis to calculate average recharge for the Edwards Aquifer in Kinney County of 86 × 10⁶ m³/yr (69,800 acre-ft/yr) for the period of analysis (1939 to 1950 and 1956 to 2001).

Sayre and Bennett (1942) noted that there is no visible natural discharge south of the three largest springs in Kinney County (i.e., Pinto, Mud, and Las Moras). They also noted that the Edwards Aquifer contains hydrogen sulfide south of the springs and that farther to the south, groundwater is highly mineralized, suggesting that groundwater movement is limited. Discharge from the Edwards Aquifer in Kinney County occurs through a variety of mechanisms and at different locations, including pumping, flowing wells, springs, subsurface flow into other hydrostratigraphic units, and as surface and subsurface flow through floodplain flow to the south. The Edwards Aquifer is interpreted to be the primary source for spring discharge, although the actual point of discharge may be another formation. It is possible that other aquifers, such as the Buda Limestone or Austin Chalk, contribute to spring discharge.

The largest-capacity spring in Kinney County, Las Moras Springs, discharges at a point where the top of the Edwards Aquifer is at a depth greater than 120 m below ground level (Welder and Reeves, 1962). Although there is faulting at Las Moras Springs, there is no indication that there is sufficient vertical structural offset to juxtapose the Edwards Aquifer with the Austin Chalk or any other permeable unit located closer to the surface. Bennett and Sayre (1962) described Las Moras Springs as an artesian spring that discharges after passing through the Grayson Shale (local equivalent of the Del Rio Clay), Buda Limestone, Eagle Ford Shale, and Austin Chalk. It is possible that the Buda Limestone and the Austin Chalk aquifers contribute to discharge at Las Moras Springs. Detailed water chemistry analysis of discharge from Las Moras Springs would help to resolve the sources of water for the springs.

Discharge at Las Moras Springs is variable, with measured flow rates that have ranged from a maximum of 1.7 m³/s recorded on 30 June 1988 to periods of no flow. The average of 155 recorded measurements for Las Moras Springs provided in Bennett and Sayre (1962) during the period of 1895 to 1965 was 21 × 10⁶ m³/yr. Using measurements from September 1939 to October 1940, Bennett and Sayre (1962) calculated the cumulative discharge from Las Moras, Pinto, and Mud Springs to be 28.4 × 10⁶ m³/yr, of which discharge from Las Moras and Pinto Springs was approximately 28.2 × 10⁶ m³/yr. Discharge by pumping and flowing wells was estimated at 4.9 × 10⁶ m³/yr for this period (Bennett and Sayre, 1962; Mace et al., 2004). Thus, discharge from the Edwards Aquifer in Kinney County by wells and spring discharge during the period 1895 to 1965 was estimated to be 33 × 10⁶ m³/yr by Bennett and Sayre (1962); see Table 1 herein.

The response of the piezometric surface and spring discharge to rainfall offers additional evidence of well-developed hydraulic connectivity from the recharge zone of the Edwards Aquifer to discharge at Las Moras Springs. Bennett and Sayre (1962) noted that fluctuations in discharge at Las Moras Springs were more correlated with rainfall at Montell and Camp Wood, located 50 and 70 km, respectively, to the northeast, than with rainfall at either Brackettville or Del Rio. Robert S. Kier Consulting (1998) noted a similar correlation in his observation that Las Moras Springs responded within a day after recharge events near Rock Springs (located 90 km northeast of Las Moras Springs) and Camp Wood. These observations support the premise that groundwater from northeast Kinney County is hydraulically connected with Las Moras Springs.

Historic pumping records are limited for Kinney County. The total discharge from wells in Kinney County during 1955–1956, which occurred during the drought of record, was estimated to be about 4.9 × 10⁶ m³/yr (4000 acre-ft/yr; Bennett and Sayre, 1962). In the absence of continuously recorded pumping data, pumpage has been estimated for Kinney County using three techniques. The first estimate of irrigation use for years the 1958–2000 was developed by the U.S. Department of Agriculture Natural Resources Conservation Service in cooperation with the Texas Water Development Board. The second estimate is a summary of historical total surface water and groundwater usage for the years 1974–2004 provided by the Texas Water Development Board ([email protected]). In the third method, annual irrigation estimates for years the 1985–2010 were developed by using irrigated crop acreage from the Texas Agricultural Statistics Service and irrigation use rates from the U.S. Department of Agriculture-Natural Resources Conservation Service. These estimates are summarized in Table 2 and graphically presented by year in Figure 7.

The three estimates of pumpage in Kinney County are consistent. In general, pumping increased in the 1960s and remained relatively high until the late 1980s before returning to somewhat lower rates in the 1990s and early 2000s. International Boundary and Water Commission data for the period 1963–2000 indicate that Las Moras Springs quit flowing in 1964, 1966, 1975, and 1991. These pumpage estimates correlate with spring discharge at Las Moras Springs. In general, Las Moras Springs only quit flowing when pumpage in Kinney County exceeded 12 × 10⁶ m³/yr.

To support the conceptual model of a separate Kinney County Pool, it is necessary to identify where the remaining recharge to the pool is discharged. Previous investigators ascertained that discharge of this volume of water from the Edwards Aquifer in Kinney County (estimated to be 86 × 10⁶ m³/yr minus 33 × 10⁶ m³/yr due to discharge by pumping and spring flow, which equals 53 × 10⁶ m³/yr) can only occur as flow in the Edwards Aquifer and that this flow is east from Kinney County to Uvalde County (Maclay and Land, 1988; Maclay, 1995) or possibly to both east and west as hypothesized by Bennett and Sayre (1962). As previously described in this paper, flow to Val Verde County to the west is impeded by a groundwater divide, and flow to Uvalde County to the east is impeded by a hydraulic structural barrier.

There are two possible explanations to explain the difference in early estimates for recharge (i.e., 86 × 10⁶ m³/yr) and discharge (i.e., 33 × 10⁶ m³/yr) calculations. First, recharge to Kinney County from the Edwards Plateau and the West Nueces River is highly variable and has not been well characterized. Recent assessment of groundwater piracy in the southern portion of the Edwards Plateau (Green and Bertetti, 2010) suggests that the West Nueces River watershed and groundwater capture areas assumed in recharge estimates may be less than estimated. If true, the amount on calculated recharge would be less than 86 × 10⁶ m³/yr. Second, it is possible that there is sufficient capacity for groundwater discharge through stream floodplains in the southern Kinney County segment to accommodate the amount of recharge from the Edwards Plateau into the Kinney County segment. Potential stream floodplains that could provide points of discharge include Las Moras Creek, Pinto Creek, and other smaller tributaries, although the flow capacity of these smaller tributaries is probably limited. Recent assessments of the capacity of subsurface flow through floodplains along the southern boundary of the Edwards Aquifer, however, indicate that these floodplains can have sufficient capacity for subsurface flow to accommodate at least a portion of the discharge from the Kinney County segment to balance the water budget (Green et al., 2008). However, it is likely that total recharge to the Kinney Pool is somewhat less than 86 × 10⁶ m³/yr.

Groundwater can transfer from the Edwards Aquifer to the stream floodplains under phreatic conditions or as flow along faults. Transfer under phreatic conditions would occur at locations where floodplains, such as Pinto Creek, cross the Edwards Aquifer. Flow along faulting, as observed at Las Moras Springs, would allow for groundwater transfer from the Edwards Aquifer to stream floodplains at locations where the Edwards Aquifer is confined and at depth. The concave shape of contours in the piezometric surface in the Kinney County segment supports the premise of significant discharge at these floodplains and springs.

The western boundary of the San Antonio segment of the Edwards Aquifer has been historically mapped to extend to a groundwater divide thought to be near Brackettville in Kinney County. A refined conceptual model of the Edwards Aquifer for Kinney County and western Uvalde County was based on an assessment of the geological structural framework, water chemistry, and hydrogeology. Based on this conceptualization, the Edwards Aquifer in Kinney County is designated as a separate segment that extends from a groundwater divide between Mud and Pinto Creeks on the west to locations where the Salmon Peak Formation of the Edwards Aquifer is virtually dewatered near the Kinney-Uvalde County line on the east. Actual flow rates in the Edwards Aquifer from Kinney County to Uvalde County are stage dependent. No flow would be expected during periods of low groundwater elevation, but limited flow from west to east is possible during periods when groundwater elevations are high. This limited hydraulic communication is interpreted as justification to designate the Edwards Aquifer in Kinney County as a separate segment.

Although elevations of groundwater are higher in eastern and central Kinney County than in Uvalde County, the structural hydraulic barrier in eastern Kinney County impedes the eastward flow of groundwater in the Edwards Aquifer from Kinney County to Uvalde County. Development of preferential flow pathways in the karstic Edwards Aquifer in Kinney County allows for a large volume of water recharged into the Edwards Aquifer in Kinney County to flow toward springs and floodplains in south-central Kinney County. Tracer tests performed in the Kinney County segment indicate that groundwater flow is to the south and southwest, thereby supporting this conceptual model (Johnson and Schindel, 2015).

Most of the West Nueces River basin is west of the structural hydraulic barrier in eastern Kinney County, supporting the premise that most of the recharge from the West Nueces River basin recharges the Kinney County segment, not the Uvalde subbasin. This interpretation is consistent with the evaluation that an average of 86.10 × 10⁶ m³/yr recharges the Kinney County segment (Mace et al., 2004). Of this, approximately 28.37 × 10⁶ m³/yr and 74.01 × 10⁶ m³/yr are discharged from Las Moras and Pinto Springs and from pumping, respectively. The conceptual model proposed here is predicated on two arguments. (1) There is less than 86.34 × 10⁶ m³/yr recharge to the Kinney Pool. (2) There is sufficient capacity for discharge from the Kinney Pool through the springs and floodplains of Pinto and Las Moras Creeks and possibly through a number of smaller creek floodplains located in the Kinney Pool to balance the water budget for the western portion of the San Antonio segment of the Edwards Aquifer. These arguments support the premise that the Edwards Aquifer forms a separate pool in Kinney County, referred to as the Kinney Pool, which extends from a groundwater divide located between Mud Spring and Pinto Spring on the west to an effective structural, hydraulic barrier near the Kinney-Uvalde County line.