Multiple Groundwater Parameters and Environmental Changes using Geospatial Techniques in the Permian Basin, Texas

This study evaluates spatial analyses of groundwater quality and environmental changes to obtain information on the groundwater contamination in the Permian Basin, Texas. Coupled with the U.S. government’s open data, these analyses can identify regions where environmental change could have potentially effected groundwater quality. A total of thirty-six wells were selected within the six counties: Andrews, Martin, Ector, Midland, Crane, and Upton. Spatial distribution maps were created for six different parameters: pH, total dissolved solids (TDS), chloride, fluoride, nitrate, and arsenic. Total groundwater quality maps incorporate all the contaminants and denote regions of poor, medium, and optimum conditions. To identify spatial changes in groundwater quality, maps were separated into two different time intervals, 19922005 and 2006-2019. We found that groundwater contamination resulted primarily from the mobilization of the contaminant from natural sources or anthropogenic activities such as chemical fertilizers. Overall, groundwater quality decreased during the study period from 1992 to 2019 as population and urban growth began to develop in the Permian Basin. This study contributes on understanding of the response of groundwater quality associated with environmental change in the Permian Basin. Therefore, this research provides important information for groundwater managements in developing plans for the use of water resource in the future for Texas. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 9 April 2020 doi:10.20944/preprints202004.0136.v1 © 2020 by the author(s). Distributed under a Creative Commons CC BY license.

3 other human contributing activities [7,8]. However, no studies have combined both parameters to identify how energy development and environmental change relate to groundwater quality.
Some studies have been undertaken to estimate the groundwater quality according to oil and gas development in the US [9][10][11][12]. Various States, like California, Wisconsin, Ohio, Minnesota, Pennsylvania, Arkansas, and Colorado have had studies discussing the changes to groundwater quality since hydraulic fracturing had been introduced to each area. Long [9] studied some challenges for groundwater quality from the oil and gas industries in the States of Wisconsin and Minnesota. Even though it is challenging to determine the impact of oil and gas activities to groundwater aquifers, there are evidences that important parameters like pH and salinity are affected [10,11]. Thus, major ions of chloride (Cl) should be monitored to ensure groundwater quality. EPA also mention that levels of total dissolved solids (TDS) can be affected by hydraulic fracturing practices [12].
Recent advances in oil recovery from unconventional reservoirs have drastically increased oil production operations in the Permian Basin [13][14][15]. The area in which this research is conducted is generally utilized for ranching, agriculture, and oil and gas production [14]. Due to the importance of groundwater in the production of these valuable commodities, maintaining stable groundwater is a necessity. As a result, a sharp increase in population and urban growth in west Texas has altered the landscape, potentially changing groundwater quality [15]. Therefore, further research on the Permian Basin, West Texas must be conducted in order to obtain a better understanding of the effects of groundwater quality over time.
The purposes of this research are to 1) describe an overview of current groundwater quality in the Permian Basin, 2) determine spatial distribution of groundwater quality parameters such as pH, TDS, chloride, fluoride, nitrate, and arsenic concentrations, and 3) provide total groundwater quality and environmental change maps from 1992 to 2019 in the study area. This research contributes to understanding of the responses of groundwater resources in the Permian Basin, Texas. Thus, this research can provide important information for groundwater resources manager in making decision and developing plans for use of the groundwater resources in the future. 4

Study Area
The study area is located in Western Texas which has a total area of 538.98 km 2 . It extends across six counties of Texas; Andrews, Martin, Ector, Midland, Crane, and Upton (Fig 1). The land cover of the study area consists mostly of developed, barren, bush, grass, and crop. The alluvial environment in which the sediments were deposited consisted of interbedded sand, silt, clay, and gravel filling prehistoric river valleys [16]. Deposition of this aquifer began during the late Miocene to the early Pliocene and formed from eastward flowing streams originating from the Rocky Mountains [13].
Groundwater originating from within the study area is captured from four aquifers: Ogallala (major), Pecos Valley (major), Edwards Trinity Plateau (major), and Dockum (minor).
The Ogallala aquifer is the largest aquifer in the United States and is a major aquifer of Texas, underlying much of the High Plains region. It consists of sand, gravel, clay, and silt and has a maximum thickness of 800 feet. The Pecos Valley aquifer is one of major aquifer in West Texas.
Water bearing sediments include alluvial and windblown deposits in the Pecos River Valley. The Edwards-Trinity Plateau aquifer is a major aquifer extending across much of the southwestern part of the state. Water quality ranges from fresh to slightly saline, and most of the groundwater is used for irrigation, municipal supplies, industrial use, and power generation. The Dockum aquifer is a minor aquifer found in the northwest part of the state. It is a sandstone aquifer and the basal member of the Dockum formation with the upper layers being predominantly siltstone and claystone.
These aquifers are a valuable source of water for ranchers, farmers, and the recovery of oil and gas in the region. The deepest groundwater well is within the Dockum at 1600 feet and the Ogallala contains the shallowest at 70 feet (Fig 2). Recharge of the aquifer occurs primarily through infiltration of precipitation. Due to the high rate of evaporation in this arid region, very little reaches the water table. The recharge rate of this aquifer is lower than the depletion rate with variations from state to state. The study area in West Texas is currently experiencing the highest depletion rate whereas certain areas have seen a drawdown of as much as 100 feet [17]. The study area is categorized as a semi-arid climate, where temperatures can drastically fluctuate throughout the day. The Permian Basin average low temperatures for January are 28° F and July high temperatures are 95 °F [17]. The region receives on average 13-18 inches of rain annually, mostly during the spring (March-May) and early fall months (September-October).
During the late summer and early fall months, moist air originating from the tropics begins to rise due to the southwestern monsoon which is the primary producer for rainfall events in west Texas [18]. With this low average rainfall, the evaporation rate is greater than the precipitation rate, resulting in a dry climate with relatively low humidity.

Groundwater quality parameters
All groundwater data was obtained from the Texas Water Development Board (TWDB) -Groundwater Database (GWDB) which is an open database provided by the US government. We collected all historically available data in our study area from TWDB-GWDB and checked the location of their wells. To apply the interpolation analysis with GIS and evaluate the spatial distribution of groundwater qualities, the observed wells must be uniformly distributed throughout the study area [7,[19][20][21]. For this reason, we finalized the total 36 wells to create an even distribution throughout the study area (Table 1).
For comparing historical changes in groundwater quality, data from 1992-2005 was correlated with data from 2006-2019. The groundwater quality parameters used for this study include pH, and total dissolved solid (TDS), chloride (Cl), fluoride (F), nitrate (NO3 -), and arsenic (As). The standard quality for water has been specified by EPA and World Health Organization (WHO) ( Table 2). The selected groundwater parameters data in the study area were 7 required to provide long-term data with a relatively dense hydrological observation network.
Each well was mapped within the study area using its provided latitude/longitude coordinate and contaminant concentration level. We followed and developed the methodology from Nas and Berktay [7].
Spline interpolation was implemented to evaluate the spatial distribution of groundwater concentrations in the map. The spline interpolation estimates values using a mathematical function that minimizes overall surface curvature, resulting in a smooth surface for the study area [21]. Once the groundwater maps were created for each different parameter, a map indicating total groundwater quality was produced. According to Nas and Berktay [7], the mosaic to new raster tool was performed by merging the six groundwater maps together while also adding all the variables. The total groundwater map from poor to optimum conditions were classified to denote regions with increasing and decreasing water quality [7,19]. The map represents a combination of the six groundwater parameters.

Examining the environmental change
The land-cover data was obtained from Texas Natural Resources Information System (TNRIS).
TNRIS provided the National Land Cover Data (NLCD) 1992 and 2011 in a raster format. The data has spatial resolution at 30m for a single pixel measures 30 meters in width and length, which is 900 meters squared. Raster data management was preformed to merge these raster data sets together. For both images to fit inside the study area, an extract by mask process was performed. This process extracts the raster cells of the imagery data and places them within the county boundaries of the study area. Additionally, we applied spatial analyses to refine the data and identify issues including edge effect, resolution change artifacts, and misclassification ( Fig   3). A majority filter was selected to smooth spatial anomalies and to provide a smoother and clearer image [21]. This research utilized seven different geographic features to describe variations from 1992 to 2011, which include water, developed, barren, forest, bush, grass, and crop (Table 3). In order to correlate between the NLCD 1992 and 2011, a reclassification and grouping of the landcover was required [22,23]. Developed classification grouped the open space, low, medium, and high intensities, such as urban settlement, transportation and industrial land. Barren classification included the bare land, rock, sand, and clay. Forest category grouped the deciduous, evergreen, and mixed forests. Bush category grouped the shrub and scrub, which is less than six meter high.
Crop category included the pasture, hay, cultivated crop. Wetland classification was removed from the 1992 and 2011 maps as their relevancy within this arid region is negligible.

pH
Municipal water treatment facilities must regulate and balance pH prior to its distribution, in order to provide optimal water. The concentration of hydrogen (H + ) and hydroxyl (OH -) ions in a liquid determine the measurement of pH. These measurements decide whether the liquid is an acid, neutral, or alkaline. For this study, it is surmised that soil pH and groundwater pH have comparable concentration levels. Within soils, pH controls the mobility of contaminants such as arsenic and fluoride. As soils become acidic, fluoride mobility increases allowing concentrations to increase [24]. Alternatively, as pH shifts becoming alkaline, arsenic mobility increases, resulting in elevated concentrations [25]. These fluctuations in pH control the concentration levels of arsenic and fluoride in groundwater and soils. The pH in the study area ranges from 6.7 -8.1 SU (Fig 4), where the deeper wells are more alkaline than the shallower wells. This increase is a result of the aquifers composition, where dissolving limestones and dolomite minerals contribute to the aquifers alkalinity.
Corrosive water (pH lower than 7) has potentially damaging effects on the municipal water treatment facility and the local homeowner. These waters have the potential to corrode pipes and destroy household appliances. The EPA recommends well waters be between 6.5 -8.5 SU to prevent these damaging effects on infrastructure [26]. Soil pH is affected by climate, temperature, and parent material [27]. In arid climates, the low precipitation results in soils that are closer to neutral or slightly alkaline due to the weathering and leaching effects of rainfall.
The weathering of parent material results in the formation of soil horizons and is the contributing factor to the soils pH.

TDS
Total dissolved solids (TDS) is a measurement of the dissolved combined content of inorganic salts and small amounts of organic matter that are dissolved in water [3]. These inorganic salts can contaminate the groundwater through anthropogenic or natural activities. Increased agricultural activity can result in higher concentrations of total dissolved solids. The adverse effects of TDS on infrastructure and taste can occur due to increase concentrations (1,000 mg/L or greater) in groundwater [26]. To prevent these effects from occurring, while also providing an acceptable taste to the user, it is recommended that the concentration of TDS not exceed 600 mg/L [28].

Chloride
The chloride contamination of groundwater and the water supply have the potential to threaten the environment. The consumer may notice the prevalence of the chloride anion within drinking water at higher concentrations, producing a salty taste. The taste thresholds can range from 200 to 1,000 mg/L and is determined by its associated cation of either sodium, calcium, or magnesium [3]. Municipal water treatment facilities require sodium chloride concentrations to remain below 250 mg/L to provide optimal water quality and prevent the bitter taste that the consumer may detect [29]. Additionally, calcium or magnesium chloride concentrations may not be detected by the consumer until they reach levels of 1,000 mg/L or greater. While chlorides in and of itself pose little threat to human health, when paired with the cation sodium, both heart and kidney diseases may arise.  degradation of the municipal water treatment facility. Contamination of groundwater with oilfield brines is predominantly associated with surface spills, where fluids can reach shallow groundwaters due to leaking tanks, flowline ruptures, or other oilfield mechanical failures [6,30]. To determine whether the groundwater has been contaminated from a spill, chloride delineations must be obtained to identify the depth that the brines have penetrated through the subsurface.

Fluoride
Fluoride (F) is essential for the maintenance and solidification of our bones and to prevent dental decay. It has beneficial effects on teeth and bones when it is present at low concentration in drinking water. Fluoride in water keeps teeth strong and reduces cavities by about 25% in children and adults. However, it may cause mottling of the teeth depending on the concentration, the age of the child, the amount of drinking water consumed, and the susceptibility of the individual [31]. The presence of fluorides in groundwater is most associated with weathering where water passing through the subsurface encounters fluorine bearing minerals [32]. Through environmental studies of fluoride in the subsurface, there is a direct correlation between mobility of pH and fluoride [24]. Soils that are more acidic allow for an increase of fluoride mobility and leaching where plant roots may subsequently accumulate the additional fluoride. This characteristic is demonstrated within the agricultural fields of southeast Martin County (Fig 7).
In this region, there is a noticeable decrease in the concentration of fluoride between the two different time intervals.  the site of this well is a cement production facility (Fig 8). Fluorspar (calcium fluoride) is a common mineralizing agent in the manufacturing of cement [33]. In the oilfield, casing cement is used to isolate oil, gas, and water zones from the wellbore while also bonding the casing to the wall. Runoff and infiltration from the production of these materials, has resulted in the elevated readings observed. Furthermore, this well passes through the confined Dockum aquifer, where groundwater has an increased residence time and natural attenuation of the contaminant is lower.

Nitrate
Nitrate (NO3 -) is a naturally occurring compound to sustain healthy plants in the ecosystem. It is in a more stable oxidation state than nitrite (NO2 -) due to its extra oxygen, resulting in nitrite being detected with increased concentrations in a reducing environment [34]. While nitrates are found naturally occurring in groundwater, elevated concentrations are typically the result of anthropogenic activities [35,36]. Contamination of the groundwater from nitrates is frequently associated with infiltration of inorganic nitrogen fertilizers and livestock waste from agricultural procedures [36]. Nitrogen fertilizers are commonly applied annually to increase the overall quality of cultivated crops and increase output. These fertilizers can reach the groundwater through nitrate leaching in the subsurface.
The Environmental Protection Agency (EPA) has established a specific MCL for nitrate at 10 mg/L and elevated concentrations pose a small threat to human health, particularly in infants [35]. Groundwater wells rarely exceed this limit unless located in regions with increased agricultural activity. Well depth is an underlying factor affecting the concentration levels of   there was a noticeable shift in concentrations (Fig 9). The concentrated animal feeding operation in northwest Andrews County experienced a reduction in nitrate levels, potentially from a decrease in operations or through better management processes. The percentage of crop increased within the study area, most notably in Eastern Martin County (Fig 12). Coupled with a shallow groundwater well depth, this resulted in nitrate levels continually increasing over time in this region. Furthermore, as the city of Midland continuously experienced urban growth, the application of inorganic nitrate fertilizers on household lawns and public parks elevated nitrate concentrations [37].

Arsenic
Arsenic is naturally found within the subsurface as a trace element on rocks or soils and is commonly used in agriculture activities. The two valence states of arsenic often found in groundwater are arsenite (As +3 ) and arsenate (As 5+ ), but concentrations rarely exceed the recommended EPA MCL of 10 µg/L [2,38]. Arsenic poisoning (arsenicosis) can occur from exposure of 50 µg/L or greater contaminated groundwater and can lead to harmful effects on the human body such as an increased risk of cancer, diabetes, and damage to internal organs [6,25].
Within alkaline environments (pH greater than 7), arsenic becomes mobilized allowing for increased concentrations to be observed. These environments promote the release of arsenic through the electrostatic repulsion of the negatively charged Fe oxides/hydroxides and arsenic compounds. Other potential inputs of arsenic in groundwater occur when petroleum hydrocarbon releases create reducing environments allowing for its mobilization [39]. Microbial activity increases the degradation of the hydrocarbons and consumes terminal electron acceptors producing these environments. Once the microbial activity has progressed and there is a reduction in the redox conditions, the concentration of the arsenic in groundwater will decrease and return to its ambient levels [39,40]. While exposure to high concentrations can be fatal, smaller concentrations (8-14 µg/L) can also lead to damaging effects such as skin lesions [38].
It's important for individuals obtaining drinking water from wells to regularly test and purify for arsenic to prevent these damaging effects.

Total groundwater quality
Water table can change over time due to changes in precipitation patterns, streamflow amount, and human-induced changes such as groundwater pumping and land development [11,41].
Changes in water table in wells are driven by the interplay between groundwater recharge and discharge to and from aquifers. In general, water tables in wells decline due to increased groundwater withdrawal and/or reduced aquifer recharge. The risk of contamination is greater for unconfined (water-table) aquifers than for confined aquifers because they usually are nearer to the land surface, and they lack an overlying confining layer to impeding the movement of contaminants. Because groundwater moves slowly in the subsurface and many contaminants sorb to the sediments, restoration of a contaminated aquifer is difficult. In unconfined aquifers, contaminants from the soil or subsurface will directly affect the groundwater quality.
A wide range of different chemicals can be dissolved in groundwater as a result of interactions with the atmosphere, the surficial environment, soil and bedrock. The mutual influence of various chemical factors helps to evaluate hydrological processes responsible for changes in the groundwater quality. Groundwater tends to have much higher concentrations of most constituents than the surface waters do, and deep groundwater that has been in contact with the rock for a long time tends to have higher concentrations of the constituents than the shallow water. Shallow groundwater consists of Ca (calcium)-Na (sodium)-HCO3 (bicarbonate) dominantly formed by the interaction between atmospherically recharged meteoric water with the soil and shallow bedrock. These waters are usually fresh but upwelling of deeper saline fluids or saline intrusions from adjacent seawater bodies can influence their chemical composition [41].
Intermediate or deep depths groundwater rapidly increase in concentration of constituents primarily by the addition of SO4 (sulfate) and Cl (chloride). The concentration of bicarbonate ions decreases because of the precipitation of mineral phases such as calcite. Local variations in chemistry and anions may be due to a variety of rock-water interactions or local processes that result in Na-SO4, Na-HCO3, and Mg-SO4 type waters. The pH begins to rise in this zone and oxygen-consuming reactions and redox mineral controls tend to lower the Eh [42].
Groundwater quality maps were created using a modified version first discussed by Ducci [19], where thematic maps were initially produced using the interpolation method and subsequently stacked developing the total groundwater quality map [7,19,24]. The modified version utilizes a summation process, combining and calculating the six contaminants together, and defining regions of poor, medium, and optimum groundwater conditions (Table 4)

Environmental change
With the advancement of horizontal drilling and hydraulic fracturing, previously untouched shale strata were now viable, greatly increasing the amount of recoverable reserves. As a result, rig count dramatically rose to nearly 500 at the end of 2011 and oil prices peaked at over $100 per barrel, ultimately allowing production to reach 1 million barrels per day [43]. Additionally, associated oilfield facilities grew to account for the increase in production. These factors resulted in a considerable change in both economic growth and land cover from 1992 to 2011 where the amount of developed land increased by 11.78 km 2 or 176% while also decreasing the percentage of barren and grass throughout the study area (Table 6).  agricultural output and quality. Global Positioning Systems (GPS) continually improved throughout the 1990's and into the 2000's, resulting in automated farm equipment that efficiently map and plan fields, increase production, and reduce the overall price from seed to harvest.
Within this arid region, the primary water source is the Ogallala aquifer, which provides the necessary groundwater vital to produce various crops. The development of center pivot irrigation in the 1950's supported farmers to establish crops in these arid regions. Furthermore, genetic engineering of crops in the mid to late 1990's, lead to plants that are more resistant to insects, weeds, and viruses [45]. These factors, coupled with a growing demand from a rising population, allowed the percentage of Crop from 1992 to 2011 to increase by 14%, primarily within Martin County. To protect groundwater quality from further human activities, government agencies and local communities should adopt the following strategies: regulating the disposal of produced fluids from oil and gas activities, controlling the amount and type of agricultural chemicals, promoting responsible use of waste drainages in the vicinity of mining areas, and boosting the development of rural and industrial infrastructures.

Conclusions
We evaluated groundwater quality parameters such as pH, TDS, chloride, fluoride, nitrate, and