Assessing Vulnerability of Regional-Scale Aquifer-Aquitard Systems in East Gulf Coastal Plain of Alabama by Developing Groundwater Flow and Transport Models
2. Methodology and Model Development
2.1. Study Site: Geology and Hydrogeology Conditions
2.2. Process-Based Numerical Models to Assess GWV
2.2.1. Geology Model for Southern Alabama Aquifers
- Gordo aquifer. The upper part of the Tuscaloosa Group is composed of the Gordo Formation, which is an important source of groundwater from northwest to east-central Alabama . The Gordo Formation has an average thickness of 300 feet, but it thickens to 500 feet downdip . A nonmarine clay layer in the upper section of the Gordo Formation serves as the confining unit above it .
- Eutaw aquifer. Provides a significant amount of water for Alabama. Outcrops range in a thickness from 100 to150 ft in the eastern part of the state and 350 to 400 ft in the western part . The Mooreville and Demopolis Chalks form the confining unit above this aquifer. The chalks extend until around Bullock County where they transition to sands and clays of the Blufftown Formation .
- Providence-Ripley aquifer. The Ripley Formation has a thickness varying from 150 to 250 feet, and the Providence Sand exhibits a thickness increasing from less than 50 feet in Lowndes County to around 300 feet at the eastern boundary of Alabama . It provides a significant groundwater source in south-central and east-central Alabama. There are 117 screened wells in the Ripley aquifer, and their depths vary from 18 ft (below land surface, or bls) in the outcrop area to 1045 ft bls downdip. The confining unit above this aquifer is made up of the Prairie Bluff Chalk, Clayton Formation, and Porters Creek Formation. Meanwhile, in the eastern part of the state, a marine clay layer located in the lower section of the Clayton Formation serves as the confining unit above .
- Nanafalia-Clayton aquifer. The thickness of this aquifer varies from 250 ft in south-central and southwestern Alabama to 75 ft in southeast Alabama . To the southwest of the state, the confining unit above consists of the Yazoo Clay. In other regions, the confining unit is made up of silt, clay, and clayey sand located near the middle of the Tuscahoma Formation .
- Lisbon aquifer. Provides the significant public, domestic, agricultural, and industrial water source for Alabama’s EGCP . The Lisbon Formation is 75~165 ft thick from east to west . The confining unit above is located near the middle of the Tuscahoma Formation. It consists of silt, clay, and clayey sand .
- Gulf-coastal lowland aquifer. This aquifer can be found in southern Mobile and Baldwin Counties and consists of clastic sediments in the Miocene undifferentiated, where a complete Miocene section exists stratigraphic interval of the Miocene section is progressively abbreviated farther north due to erosion . Where present, the entire Miocene thickness ranges from less than 50 to approximately 2500 ft . The confining unit above this aquifer in the southwestern region of the state is the Yazoo Clay. However, in the south-central and southeastern parts, the Yazoo Clay transitions into the Ocala Limestone toward the east. In these regions, the confining unit is a gray clay that is dense and has a soft texture .
|Coker aquifer||Cross-bedded sand, light-colored micaceous, very fine to medium sand, and varicolored micaceous clay . Deposited during a time of marine transgression, the Coker Formation was deposited .|
|Gordo aquifer||Cross-bedded sand, gravelly sand, and lenticular beds of locally carbonaceous clay that are partially mottled moderate-red and pale-red-purple. Pale-yellowish-orange, poorly sorted, cross-bedded gravelly fine to very coarse quartz sand, containing irregular beds of moderate-reddish-brown to pale-red-purple sandy clay . The boundary between marine sediments of the Coker Formation (which consists of massive, marine clay with thin beds of fine-grained sand) and nonmarine sediments of the Gordo Formation formed during a period of significant sea level regression .|
|Eutaw aquifer||The western part is described as light-greenish-gray well-sorted micaceous cross-bedded fine to medium sand. The eastern part is described as light-greenish-gray to yellowish-gray, well-sorted, micaceous, partly fossiliferous fine to medium quartz sand interbedded with dark-gray carbonaceous clay, greenish-gray micaceous sandy clay, and thin beds of glauconitic, fossiliferous sandstone . The Eutaw Formation was primarily formed in a marginal marine setting connected to a barrier island and deltaic environment .|
|Providence-Ripley aquifer||The eastern part consists of light gray to pale-olive massive, micaceous, glauconitic, fossiliferous fine sand, sandy calcareous clay, and thin indurated beds of fossiliferous sandstone. The western part of this formation contains micaceous fine to medium quartz sand, cross-bedded in the upper part, and sandy calcareous clay . Gray, fossiliferous, silty Demopolis chalk in central and western Alabama. The chalk overlies the Mooreville Chalk and grades into the Blufftown and Ripley Formations in the east of the region .|
|Nanafalia-Clayton aquifer||Clayton Formation in eastern Alabama comprises fine sand, medium-gray silty, calcareous clay, sandy fossiliferous limestone, and gravelly, medium to coarse sand containing clay pebbles. Glauconitic sand, massive clay, and fossiliferous sandy clay form the Nanafalia Formation [49,50] The aquifer includes the basal sand of Tuscahoma Formation, and the whole of the Nanafalia and equivalent Baker Hill (in eastern Alabama), Naheola, Porters Creek, and Clayton Formations. However, one or more of these formations is absent at any one geographical location. The aquifer consists mostly of unconsolidated sand and clay beds, but locally includes carbonate rocks .|
|Lisbon aquifer||Sand, limestone, and sandy limestone, highly fossiliferous, glauconitic, quartz sand, and lenses of greenish-gray clay . According to Toulmin and LaMoreaux (1963) , the Lisbon Formation in southeast Alabama consists primarily of sand but also contains significant amounts of limestone and sandy limestone. The Gosport Sand is only mapped in the west and central Alabama, between the Alabama River and the Alabama-Mississippi state line, and is comprises highly fossiliferous, glauconitic, quartz sand and lenses of greenish-gray clay , with an outcrop thickness ranging from 17 to 30 feet .|
|Gulf-coastal lowland aquifer||Clay, silt, sand, and gravel, with subordinate limestone and lignite beds .|
2.2.2. Groundwater Flow Model Built by MODFLOW
2.2.3. Contaminant Transport Model Using MODPATH
3. Results of Flow and Transport Models
3.1. Steady-State Groundwater Flow Model
3.1.1. Model Calibration
3.1.2. Parameter Sensitive Analysis
3.2. Groundwater Age and Residence Time
3.2.1. Backward Particle Tracking to Calculate Groundwater Age
3.2.2. Forward Particle Tracking Calculates Residence Time
3.2.3. Total Travel Time
4.1. Vulnerability of Southern Alabama Aquifers with Broad Groundwater Ages
4.2. Extension to Transient Flow: Impact on GWV
Data Availability Statement
Conflicts of Interest
Appendix A. Local Scale Result
Appendix B. Wavelet Analysis
|Scale||<30 Days||30–90 Days||90–180 Days||180–365|
|Scale||<30 Days||30–90 Days||90–180 Days||180–365 Days||>365 Days||All Scales|
Appendix C. Vadose Zone Age and Model Parameter Table
|Name||Explanation||Parameters||Unit||Original Range||Calibrated Distribution|
|Horizontal Hydraulic Conductivity||Gulf coastal lowland aquifer||HK_100—sc1v1 to HK_100—sc1v6||ft/day||1~42,000||1~35,643|
|Lisbon aquifer||HK_200—sc3v1 to HK_200—sc3v11||0.00833~1000||0.0789~1000|
|Nanafalia-Clayton aquifer||HK_300—sc4v1 to HK_300—sc4v7||0.01~5000||0.0316~5000|
|Providence-Ripley aquifer||HK_400—sc5v1 to HK_400—sc5v9||0.0567~5000||0.0567~4546|
|Eutaw aquifer||HK_500—sc6v1 to HK_500—sc6v8||0.0255~42,000||0.0365~28,293|
|Gordo aquifer||HK_600—sc7v1 to HK_600—sc7v5||0.0255~42,000||0.0255~22,481|
|Coker aquifer||HK_700—sc8v1 to HK_700—sc8v18||0.0255~42,000||0.0255~219|
|Clay||HK_1100—sc2v1 to HK_1100—sc2v5||1 × 10−6~1||0.000105~0.198|
|Recharge rate||Zone 1||RCH_10—sc10v1 to RCH_10—sc10v8||ft/day||1 × 10−8~1||1.3 × 10−7~0.0338|
|Zone 2||RCH_20—sc9v1 to RCH_20—sc9v8||1 × 10−8~5||6.9 × 10−7~0.0280|
|Zone 3||RCH_30—sc11v1 to RCH_30—sc11v8||1 × 10−8~6||8.8 × 10−6~0.0210|
|Zone 4||RCH_40—sc12v1 to RCH_40—sc12v5||1 × 10−8~7||3.9 × 10−8~0.0182|
|Riverbed Conductance||Tombigbee River||RIV_11 to RIV_18||ft2/day||0.0001~100||0.00130~9.97|
|Alabama River||RIV_21 to RIV_27||0.0001~100||0.00453~64.87|
|Cahaba River||RIV_31 to RIV_34||0.0001~100||0.0289~44.46|
|Conecuh River||RIV_61 to RIV_67||0.0001~100||0.0582~3.09|
|Pea River||RIV_71 to RIV_75||0.0001~100||0.0217~8.74|
|Choctawhatchee River||RIV_81 to RIV_84||0.0001~100||0.113~0.14|
|Sipsey River||RIV_91 to RIV_93||0.0001~100||0.00760~4.17|
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Ponprasit, C.; Zhang, Y.; Gu, X.; Goodliffe, A.M.; Sun, H. Assessing Vulnerability of Regional-Scale Aquifer-Aquitard Systems in East Gulf Coastal Plain of Alabama by Developing Groundwater Flow and Transport Models. Water 2023, 15, 1937. https://doi.org/10.3390/w15101937
Ponprasit C, Zhang Y, Gu X, Goodliffe AM, Sun H. Assessing Vulnerability of Regional-Scale Aquifer-Aquitard Systems in East Gulf Coastal Plain of Alabama by Developing Groundwater Flow and Transport Models. Water. 2023; 15(10):1937. https://doi.org/10.3390/w15101937Chicago/Turabian Style
Ponprasit, Chaloemporn, Yong Zhang, Xiufen Gu, Andrew M. Goodliffe, and Hongguang Sun. 2023. "Assessing Vulnerability of Regional-Scale Aquifer-Aquitard Systems in East Gulf Coastal Plain of Alabama by Developing Groundwater Flow and Transport Models" Water 15, no. 10: 1937. https://doi.org/10.3390/w15101937