1. Introduction
Groundwater accounts for approximately 32 percent of all water supplied by municipal water treatment facilities. As population continues to rise, there will be a heavier reliance on this diminishing natural resource. The Safe Water Drinking Act (SWDA) was originally established by Congress in 1974, and developed groundwater quality regulations to protect over 150,000 public water systems across the U.S. [
1]. Citizens obtaining their water from privately owned water wells in rural areas not serviced by local water treatment facilities should take preventative steps such as testing and decontamination prior to consumption, as these wells are not regulated by the Environmental Protection Agency (EPA).
Once the EPA has determined that a contaminant poses a risk to water quality, they will develop a maximum contaminant level goal (MCLG). The maximum contaminant level goal is achieved when the most vulnerable individuals, such as infants, children, and the elderly, would not experience adverse health effects from exposure of the contaminant [
2,
3]. After the EPA has developed a MCLG for a contaminant, then the agency sets a maximum contaminant level (MCL). The EPA enforces MCL’s within the municipal water treatment facilities to maintain water quality but in order to enforce the MCL, they must be both economically and technologically viable [
1,
4]. The three parameters that are not controlled by the EPA in this study are chloride, pH, and total dissolved solids (TDS). The remaining three contaminants, arsenic, fluoride, and nitrate have specific MCLG/MCL values determined by the EPA.
Numerous studies have been conducted to identify the effect of oil and gas production on environmental changes. Unconventional oil and gas production provides a decreasing trend of groundwater quality (chloride, nitrate) over time in the Permian Basin, West Texas [
5,
6]. The land-use changes in West and Central Texas during the shale boom of 2008–2012 are a direct result from the utilization for unconventional reservoirs and the development of energy resources and other human contributing activities [
7,
8]. Previous studies [
5,
6,
7,
8] have suggested that oil and gas development affected various entities, including groundwater quality and land-use changes. However, few studies have combined both groundwater and land-use factors to identify how oil and gas development related to the environmental changes.
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 on 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.
2. 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 (
Figure 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 among the major aquifers 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 (
Figure 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 the Permian Basin, West Texas is currently experiencing the highest depletion rate, whereas certain areas have seen a drawdown of as much as 100 feet [
17]. As for the hydraulic characteristics, average transmissibility are 365 feet
2/day, and the storage coefficient was 0.074. Lateral movement of ground water from the Ogallala likely occurs in the northern edge of the region. Hydraulic fracturing is also known to affect groundwater reservoirs. During hydraulic fracturing, various chemicals are injected underground site to generate fractures and increase the production of hydrocarbons in reservoirs that have low-porosity and low-permeability [
6].
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.
5. Conclusions
We evaluated groundwater quality parameters such as pH, TDS, chloride, fluoride, nitrate, and arsenic from 1992–2005 and 2006–2019 and identified land cover maps where specific changes in the environment effected groundwater quality in the Permian Basin, Texas. Utilizing advanced geospatial techniques, these parameters described areas from optimum and poor groundwater conditions. Factors that contribute to the level of contaminants in groundwater are natural sources, anthropogenic activities, and aquifer depth. The mobilization of arsenic and fluoride from natural sources resulted primarily in the fluctuations of the subsurface pH. These alterations in pH directly resulted in varying concentrations of contaminates within the groundwater. Anthropogenic activities such as petroleum spills and inorganic chemical fertilizers contributed to the contaminant load in groundwater. An underlying factor for the contamination is aquifer depth, where contaminants may reach shallower unconfined aquifers quickly, yet have a decrease in residence time. Developed and crop land cover significantly rose from NLCD 1992 to NLCD 2011 due to the increase in production from unconventional natural resources and advances in crop management. These increases consequently resulted in a decrease in overall groundwater quality in the Permian Basin, Texas. Total groundwater quality maps demonstrate a transition of water quality related to the advancement of urban development and population. This research provides significant information for the management of groundwater resources and the response to these potential changes in the Permian Basin, Texas.