The Impact of Hydraulic Fracturing on Groundwater Quality in the Permian Basin, West Texas, USA
Round 1
Reviewer 1 Report
It is an important subject and data provided by this manuscript are interesting and useful to readers. However, there are a number of things requring clarification. They are given below 1. Please explain how a well can be considered an aleatory well (Line 135) 2. The relationship between depth and water quality parameters (Fig. 6) is based on data in 2016. Are data of others years show the same trend? 3. In additon to hydraulic Fracturing, any other soutces of contamination to shallow groundwater? In other words, rationale and evidence is needed to attribute the detected contamination to hydraulic fracturing. 4. Explain the structure of the aquifer in the target area 5. In Fig. 1, only five groundwater wells in Ector County are shown, not 6 as described in the text (line 138)Author Response
Responses to the Reviewers (water-688901)
We have made careful and extensive modifications on the revised manuscript to address the concerns and comments made by the reviewers and the editor.
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Reviewer 1
It is an important subject and data provided by this manuscript are interesting and useful to readers. However, there are a number of things requiring clarification. They are given below
Comment 1-1
- Please explain how a well can be considered an aleatory well (Line 135).
Our response
We agree with the reviewer’s comment. In this revision, we clarified the reason which wells were selected from the dataset and why. Additionally, we described how the dataset of well analysis was explained.
To understand the distribution of chemical fluids, the wells must be evenly distributed throughout the study area. First, we selected historically available wells from the Texas Water Development Board (TWDB) - Groundwater Database (GWDB). Next, we checked the locations of wells (i.e. latitude/longitude), and then, finalized some wells that are uniformly distributed over the whole study area. For this reason, we selected the 30 wells per county to create an even distribution throughout the study area as mentioned in page 7.
Page 7: “We collected all historically available data in the study area and checked the location of their wells. To understand the distribution of chemical fluids, the wells must be uniformly distributed throughout the study area. For this reason, we finalized the 30 wells per county to create an even distribution throughout the study area (Figure 1). The data on selected parameters were required to provide long-term data with a relatively dense observation network. Each well was mapped within the study area using its provided latitude/longitude coordinate and contaminant concentration level.”
Comment 1-2
- The relationship between depth and water quality parameters (Fig. 6) is based on data in 2016. Are data of others years show the same trend?
Our response
Our study area is the counties of Ector, Midland, and Martin. Each county has different numbers for groundwater and hydraulic fracturing wells along with the different time ranges for starting and ending years. For example, we evaluated 5 groundwater wells in Ector county, 10 groundwater wells in Midland county and 6 groundwater wells in Martin county, as we described in the section about the study area. In terms of hydraulic fracturing wells, each county has different time ranges for starting and ending years; Ector county is available from 1999, 2003, 2007 to 2016, Midland county is available from 1998, 2007, 2016, 2019, and Martin county is available only 2016. The 2016 data is only available to show the relationship between depth and water quality parameters. Therefore, we finalized that 2016, which has long records with relatively dense observation in the study area, is the most up-to-date year to collect hydraulic fracturing information. In this revision, we clarified the depth of the respective wells related to their chemical composition on page 19 following the reviewer’s comment.
Page 19: “In general, data of other years show relatively similar trends in Figure 6. Water depth can change over time due to changes in precipitation, streamflow amounts, and human-induced changes such as groundwater pumping and contamination [49]. The risk of contamination is greater in unconfined aquifers than confined aquifers because they usually are nearer to the land surface, and they lack an overlying confining layer 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. 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.”
Comment 1-3
- In addition to hydraulic Fracturing, any other sources of contamination to shallow
groundwater? In other words, rationale and evidence is needed to attribute the detected contamination to hydraulic fracturing.
Our response
We agree with the reviewer. In this revision, we explicitly mentioned other sources of contamination to shallow groundwater with hydraulic fracturing.
Page 21-22: “Groundwater contamination is the most severe and controversial concerns of hydraulic fracturing development. Oil and gas development in the Permian Basin has increased dramatically since 2010s. Some studies suggested that the hydraulic fracturing could lead to additional contaminations [56-58]. Fluids may leak from faulty casing and cementing, thus contaminating shallow aquifers. The causes of shallow fluid leakage and transport include casing damage which provides leaking channel and faulty cementing which provide flowing space. The water samples from drinking wells less than 1km from shale gas wells have higher methane concentration [56]. In addition, damaged casing and flawed cementing could lead to natural gas leaking into shallow groundwater. Flowback contains multiple contaminants with complex composition, mishandling of which would lead to contamination. Further research is required to find out how the hydraulic fracturing activities would affect the transport of naturally occurring hazardous substances.”
Reference
- Jackson R B, Vengosh A, Darrah T H, et al. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. Proceedings of the National Academy of Sciences, 2013, 110(28): 11250-11255.
- Spellman, F.R. Environmental impacts of hydraulic fracturing. Taylor & Francis Group 2013, p75.
- Zhang, D.; Yang, T. Environmental impacts of hydraulic fracturing in shale gas development in the United States. Petroleum Exploration and Development, 42, 2015, 876-883.
Comment 1-4
- Explain the structure of the aquifer in the target area
Our response
We agree with the reviewer’s comment. In this revision, we explained the structure of aquifers in the study area on page 5-6.
Page 5-6: “Groundwater originating from within the study area is captured from four aquifers: Ogallala (major), Pecos Valley (major), Edwards Trinity Plateau (major), and Dockum (minor) [18]. 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.”
Comment 1-5
- In Fig. 1, only five groundwater wells in Ector County are shown, not 6 as described in the text (line 138)
Our response
Yes, we agree with the reviewer’s comment. As the reviewer suggested, we modified Table 1 and the text (line 138; page 7-8), and collected the information from only five groundwater wells. We removed the well number “2758901” that is located to next to “2758902”.
Reviewer 2 Report
The manuscript deals with the impact caused by fracturing to groundwater bodies in Texas, USA. The paper is very well structured. The introduction is informative, providing an overview od the scientific problem addressed and info from other relevant researches. The methodology is sound and presents satisfactory results thus, leading to sound interpretation. I think overall the paper is very good and, frankly, i hardly can find any flaws.
For the sake of improvement, I would suggest the authors provide more details about the hydrogeological regime of the area (e.g. water level, hydraulic parameters, potential lateral crossflows, etc).
I think the paper can be published in its current form.
Congrats.
Author Response
Responses to the Reviewers (water-688901)
We have made careful and extensive modifications on the revised manuscript to address the concerns and comments made by the reviewers and the editor.
*****************************************************************************
Reviewer 2
The manuscript deals with the impact caused by fracturing to groundwater bodies in Texas, USA. The paper is very well structured. The introduction is informative, providing an
overview od the scientific problem addressed and info from other relevant researches. The methodology is sound and presents satisfactory results thus, leading to sound
interpretation. I think overall the paper is very good and, frankly, i hardly can find any flaws.
Comment 2-1
- For the sake of improvement, I would suggest the authors provide more details about the hydrogeological regime of the area (e.g. water level, hydraulic parameters, potential lateral
crossflows, etc). I think the paper can be published in its current form.
Our response
I agree with the reviewer’s suggestion. In this revision, we provided more details about the hydrological regime of the study area, such as water level, hydraulic parameters, and potential lateral crossflows in page 6-7.
Page 6-7: “The 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 ft and the Ogallala contains the shallowest well at 70 ft. Average recharge of the aquifer is calculated to be 30,000 acre-feet per year and 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 is currently experiencing the highest depletion rate, whereas certain areas have seen a drawdown of as much as 100 feet [16]. Hydraulic characteristics that influence the effectiveness of an aquifer include transmissivity and storage coefficient. Average values for these characteristics are 365 square feet per day for transmissivity and 0.074 for storage coefficient [18]. Lateral movement of ground water from the Ogallala to the Trinity aquifer likely occurs along the northern edge of the region where the two formations abut.”
Round 2
Reviewer 1 Report
The manuscript was well improved.
