The groundwater division of the Texas Water Development Board supervises the measurement and monitoring of groundwater levels and chemical compositions in more than 100,000 spring and well locations in Texas. This information is stored in the database of the Water Information Integration and Dissemination (WIID) system of the board and is readily available to the public (www.twdb.texas.gov). Annually, measurements of some specific physico-chemical parameters of groundwater are conducted in about 8000 wells (this may vary depending on the socio-economic activities in the areas and/or the quantity of groundwater withdrawal in each county) throughout the state. The database of the TWDB also indicates the water-use category (industrial, commercial, unused, livestock, irrigation, public supply, domestic, etc.) for each well.

To assess the groundwater suitability for irrigation, some 127 groundwater samples were collected from different borehole wells in the study area by TWDB. 36 water samples were collected in Andrews and Gaines counties, while 28 locations were sampled in Martin County. A total of 10 and 17 samples, respectively, were collected from boreholes in Howard and Midland counties. The geographic coordinates of each well location are clearly indicated in the TWDB database, and the spatial coverage of the sampled sites is displayed in Figure 2. Before sample collection, the water in each well was pumped for 15 minutes, after which onsite measurements of temperature (˚C) and pH were conducted. The concentration values of major cations (Ca²⁺, Mg²⁺, and Na⁺) and TDS (Total Dissolved Solids) were measured in the laboratory using the standard procedures outlined by the American Public Health Association (APHA) and approved by the TWDB. The concentration data of major cations, TDS, pH, temperature, sodium absorption ratio (SAR), and % Na were calculated/measured by the water quality department of the TWDB for the period from 2004 to 2024. We then used these data to calculate relevant irrigation water quality indices such as Kelly’s ratio (KR), magnesium absorption ratio (MAR), and soluble sodium percentage (SSP) using the expressions given in Table 1, according to established methods for evaluating irrigation water quality.

Table 1. Equations applied in computing some relevant irrigation water quality indices.

4.2.1. Sodium Adsorption Ratio (SAR)

Sodium Adsorption Ratio (SAR) is used to evaluate the potential threat to agricultural crop growth and productivity posed by the presence of sodium in groundwater systems used for irrigation ([18]). Elevated SAR concentration values in irrigation water samples facilitate the adsorption and binding of sodium molecules on soil particles, which results in the breakdown of soil structure. Consequently, the soil becomes hard and compacted when dry, thereby making it impermeable to water ([50]). SAR, expressed in milliequivalents per litre, is used to assess the relative concentration of sodium compared to magnesium (Mg) and calcium (Ca) in groundwater samples. The replacement of magnesium and calcium increases the concentration of sodium in irrigation water, which leads to the saturation of the ion-ex complex and dispersion of clay particles, thus degrading the soil’s structure and composition ([40]). SAR values of the water samples from the counties have been computed by the TWDB.

4.2.2. Percent Sodium (Sodium Hazard)

Irrigation water is also classified based on the percentage of sodium (%Na). It is a crucial parameter, which, when low in value, also reduces the risk of water infiltration and aeration problems ([32]). Irrigation water with a significantly low risk of water penetration problems is excellent for crop growth. Areas where irrigation waters of high to moderate %Na risk are employed in crop cultivation are commonly prone to loss of soil permeability/structure and usually require management interventions ([9]). Values of %Na of groundwater samples collected in the study area have been calculated by the TWDB using the method of [51].

4.2.3. Kelly’s Ratio (KR)

This indicator is essential for evaluating the quality of irrigation water by determining the threat posed by the water to soil degradation due to salinity and sodicity issues. In irrigated agriculture, KR values that are greater than unity suggest high water sodicity, indicating that the water is unsuitable for irrigation activities due to potential alkali hazards (Table 2) ([5]). This index is calculated using the expression provided in Table 1 above.

Table 2. Classification of groundwater for irrigation use (adopted from [15]).

4.2.4. Magnesium Adsorption Ratio (MAR)

MAR is one of the relevant qualitative parameters for assessing the suitability of groundwater for irrigation. Increased magnesium levels in irrigation water are mainly caused by the high sodium exchange capacity of irrigated soils, which affects the uptake of essential nutrients ([33]; [6]). According to [25] and [24], excessive magnesium levels in irrigation water reduces crop yield and adversely impacts plant growth by increasing the alkalinity of the soil. This parameter was determined for all collected samples using the method of [49] (Table 1).

4.2.5. Soluble Sodium Percentage (SSP)

This is a measure employed in evaluating the suitability of water for irrigation, specifically by assessing the potential for sodium to adversely impact soil properties ([1]). It reflects the proportion of sodium ions relative to other cations in the water. Elevated SSP values can result in reduced soil permeability and negative effects on plant growth. The formula used in computing this index is expressed in Table 1.

The meaning, adopted values, and class range of each of these parameters are given in Table 2. Classification of groundwater quality for irrigation in each county was carried out based on these class ranges.

In this study, we used a Geographic Information System (GIS) software, precisely Surfer version 23.0 software package. This tool expedited the production of 2D models showcasing the spatial distribution of the various irrigation groundwater quality indices that are relevant to this study. The distribution and variability of groundwater quality are visually represented in these maps in detail, which enhances our understanding of the suitability of groundwater for irrigation use in the study area.

With respect to the relative concentrations of cations in the irrigation water, assessment of percent sodium (%Na) in the groundwater samples revealed a worrisome range (12 - 70.44) exceeding permissible limits (60%) in 9% of the samples, all of which were taken from Andrews, Howard, and Martin counties (Table 2). Table 3 shows maximum %Na of 62%, 70.44% and 66% for Andrews, Howard, and Martin counties, respectively. Elevated levels of %Na are, therefore, a serious concern mainly in Andrews County and partly in Howard and Martin counties. Figure 3 shows %Na hotspot in the eastern and southern parts of Andrews County, suggesting that groundwater in these areas is not fit for irrigation purposes. The southeastern and northern portions of Martin and Howard counties, respectively, show small zones with slightly elevated %Na levels that could compromise the integrity of groundwater for irrigation use in these areas. This poses a great risk to crop productivity and soil health as Na bridges the pore throat, thereby reducing soil permeability, which impedes nutrient uptake and water movement ([38]; [12]).

Previous studies suggest that fertilizer use and/or carbonate mineral dissolution within the Ogallala aquifer could be the potential contributing factors ([7]). According to [20], carbonate combined with excessive Na also increases the alkalinity of groundwater and could be probably responsible for the slightly higher pH values recorded in about 65% of the water samples. Notwithstanding, based on %Na values, 91% of the groundwater samples fall within the allowable limit (<60%) required for irrigation use (Table 2).

SAR values computed for the groundwater samples were found to range from 0.49 to 13.34 meq/L, implying that all the samples are good for irrigation agriculture (Table 3). The results show that 99.64% of the water samples fall in the “Excellent” range, while the remaining 2.36% fall in the “Good” range (Table 2). It is worth noting that areas with the highest values of SAR coincide with zones of elevated %Na (see Figure 3 and Figure 4). This makes it imperative for the crop-specific requirements of SAR to be considered before utilizing the groundwater for irrigation purposes in the southeastern and northern parts of Martin and Howard counties, where elevated concentrations of %Na were recorded.

Figure 3. Spatial distribution map of %Na in groundwater samples collected in the study area.

Figure 4. Spatial distribution map of SAR in groundwater samples collected in the study area.

Overall, assessment of KR values, a critical index for the evaluation of irrigation water quality in agricultural provinces, revealed that 70.87% of the water samples are safe and fit for irrigation utilization (Table 2). However, 24.41% show marginal characteristics, while 4.72% are unsafe for use in irrigation agriculture due to alkaline hazards. This suggests that not all the water samples have good potential for irrigation use. According to the classification outlined by [22] and [33], who proposed this parameter, groundwater with KR values exceeding unity is considered unsafe for irrigation purposes, while those with KR values less than 1 are deemed fit for irrigation utilization. Similar classifications have also been used by several authors in literature ([38]; [52]; [4]; [45]). Using the same classification, it implies that 29.13 % of the water samples collected in the study area are not recommended for irrigation. Specifically, 10 groundwater samples from Andrews County, representing 27.77% of the total samples collected in the county, have values above 1.0 and are unsuitable for irrigation purposes. Other areas with KR values greater than 1.0 make up 17.86% and 20%, respectively, of water samples taken from Martin and Howard counties and are considered undesirable for irrigation agriculture. These affected areas are located in the southeastern part of Andrews County, the northwestern and southeastern parts of Martin County, and the northern and northeastern axis of Howard County (Figure 5). These areas are indicated in Figure 5, which shows the spatial distribution of Kelly’s ratio in the five counties.

Figure 5. Spatial distribution map of Kelly ratio in groundwater samples collected in the study area.

Generally, 25.98% of the total samples pose a significant threat to soil structure and crop productivity based on MAR values (Table 2). Invariably, this indicates that only 74.02% of the samples, the majority of which were collected in Midland County, are appropriate for irrigation agriculture (all samples taken from Midland County reported MAR values < 50 and are all suitable for irrigating crops). Excessive magnesium is indicated in 25% (9 samples), 41.67% (15 samples), 20% (2 samples), and 28.57% (8 samples) of the samples analyzed in Andrews, Gaines, Howard, and Martin counties, respectively. This poses a significant magnesium hazard to soil structure when using the MAR index. Besides anthropogenic discharges, the dissolution of dolomite minerals and carbonate cements in the Ogallala aquifer is a major contributor to the unacceptable levels of MAR in the counties. These affected areas include the northern and western parts of Gaines County, the southeastern parts of Andrews County, the northern and northwestern axis of Martin County, and the western corner of Howard County (Figure 6). Based on MAR values, Table 2 indicates that 34 samples analyzed in these areas have a deleterious impact on soil composition, which is inimical to soil fertility and crop yield.

Figure 6. Spatial distribution map of MAR in groundwater samples collected in the study area.

SSP values range from 18.95 to 78.13% (Table 3). Of these, 0.79 % (1 sample) was classed as “Excellent”, 37% (47 samples) was classed as “Good”, and 51.18% was classed as “Permissible” (Table 2). No sample recorded SSP values above 80, which implies that all the samples are appropriate for irrigation agriculture. However, 14 water samples representing 11.02% of the total samples were classed as “Doubtful” (60 ≥ SSP ≤ 80). The spatial distribution map of SSP shows that the doubtful areas are located southeast of Andrews County, west and southeast of Martin County, and north and west of Howard County (Figure 7). These values highlight the need for continuous monitoring of groundwater in these areas and the importance of adjusting agricultural practices to mitigate any harmful effects on crops.

Figure 7. Spatial distribution map of SSP in groundwater samples collected in the study area.

In irrigated areas, measurements of TDS values in groundwater samples are essential to determine the rate of water uptake by crops and ensure that plant growth rates are not reduced. Based on the laboratory classification criteria of the US Department of Agriculture, various concentration levels of TDS in groundwater will have varying degrees of impact on the irrigation suitability of waters. The irrigation suitability of groundwater based on TDS is grouped as “Preferred for Irrigation” (<150 mg/l), Slightly to Moderately Preferred” or “Good to Doubtful” (150 - 1500 mg/l), or “Unsuitable for Agricultural Use” (>1500 mg/l). Although the results of this classification of irrigation water quality indicate that no sample was fit or preferred for irrigation, 42 samples (33.07%) were deemed unsuitable for agricultural utilization, while 85 samples (66.93%) were doubtful for irrigation use and show limited irrigation potential. The spatial distribution plot of TDS measurements of the samples show that majority of the unfit locations lie in the southeastern parts of Andrews County, the northwestern and southeastern parts of Martin County and the northern part of Midland County (Figure 8). A salinity problem will likely persist at the root zones of crops in these areas which could result in water stress for a prolonged period of time thereby leading to reduced crop growth.

While the parameters successfully assessed groundwater irrigation suitability; however, the analysis is limited by the wide temporal range, which obscures potential year-to-year variations in water quality. This study evaluated the overall water quality and its suitability for irrigated agriculture. The scope did not permit distinguishing anthropogenic and geogenic influences, which is a potential area for future investigation.

Figure 8. Spatial distribution map of TDS in groundwater samples collected in the study area.