1. Recharge and Hydrochemical Evolution in Regional Sedimentary Aquifers Alan Fryar, Sunil Mehta, Estifanos Haile Earth and Environmental Sciences, University of Kentucky

2. Introduction Regional aquifers are valuable for water supply and as paleoenvironmental archives Mechanisms of recharge and controls on water chemistry vary with: –landscape position –land use –climate –aquifer stratigraphy

3. Overview Compare mechanisms of recharge and controls on water quality in two regional aquifer systems Similarities: –comprised of Tertiary fluvial sediments beneath plains –study areas located at ~ 35 to 36°N in central USA –major land use: irrigated agriculture Differences: –climate: semi-arid vs. humid –hydrostratigraphy: unconfined vs. confined aquifer

4. (http://capp.water.usgs.gov/gwa/gwa.html)

5. Approach Sample groundwater for: –field parameters (redox indicators, pH, T, alkalinity) –ions and metals –stable and radiogenic isotopes Develop conceptual hydrostratigraphic models Determine equilibrium relationships, reactions, and mixing along flow paths Numerically simulate recharge and flow

6. (http://co.water.usgs.gov) High Plains

7. Background High Plains (Ogallala) aquifer largest in USA Local water-level declines >30 m since 1930s Numerous physically-based studies of rates and sources of recharge Relatively few hydrochemical studies

8. (Keller, 2008)

9. Previous findings: Southern High Plains Recharge is focused beneath playas (ephemeral wetlands that collect runoff) Recharge is negligible in upland soils except where ponding occurs Groundwater chemistry reflects: –evapotranspiration –carbon cycling in the vadose zone –silicate weathering

10. Variety of activities –agriculture: irrigated grain, animal feedlots, grazing –petroleum production –industry (e.g., petrochemical factories) Playas retain runoff from irrigation and feedlots, industrial wastewater, sewage Southern High Plains land use

11. (Fryar et al., HJ, 2001)

13. (http://agnewsarchive.tamu.edu/dailynews/stories/WFSC/photos/Jul1207a.htm)

15. Large Cl - plumes in High Plains aquifer

16. Objectives (1) Identify sources of recharge and salinization (2) Constrain travel-time estimates through unsaturated zone and along aquifer flowpaths (3) Identify plausible reactions in the vadose and saturated zones

17. Methods—sampling Water samples collected from: –43 water wells (29 in High Plains aquifer, 10 in perched aquifers, 4 in Upper Permian units) –3 oil wells in Pennsylvanian granite wash and Permian (Wolfcamp) brown dolomite Sediment cores collected from 4 playa basins and 2 upland sites

18. Methods—modeling Determined solute speciation and saturation indices, modeled chemical evolution and mixing in High Plains aquifer (NETPATH, PHREEQC, PHRQPITZ) Modeled steady-state flow and solute transport along 200-km transect (SUTRA)

19. Results Major-ion facies is Ca-HCO 3 or Ca-Mg-HCO 3 except where contaminated or affected by salinization –consequence of carbon cycling in shallow subsurface Groundwater is oxic (O 2 > 3.2 mg/L) except for oil- field waters –denitrification in anaerobic zones above water table

20. (Fryar et al., JCH, 2000)

22. Results (continued) δD and δ 18 O trends suggest: –limited evaporation during recharge –plume waters were recharged in cooler climate Bivariate plots and numerical models indicate brine leakage from Upper Permian units into Ogallala Fm. –flow paths 10–10 2 km long, plume TDS > 3000 mg/L, discharge 10 -4 –10 -5 m 3 /d –salinization is not a result of petroleum extraction

23. (Fryar et al., HJ, 2001)

24. (Mehta et al., AG, 2000)

27. Mississippi Valley

28. Background Confined Wilcox aquifer: part of Gulf Coastal Plain aquifer system –recharged along margin of Mississippi Embayment –cross-formational leakage? Less exploited than overlying units (Mississippi Valley alluvium and Claiborne Group) –mainly used for municipal wells –local water-level declines in Memphis area

29. SE Missouri and NE Arkansas between the St. Francis and Mississippi Rivers Mixed land use: primarily irrigated crops (rice, corn, soybeans, cotton), other agriculture, some industry, small communities Study area

31. (Williamson and Grubb, 2001)

32. Objectives (1) Identify major processes controlling chemical composition of groundwater in the Wilcox aquifer (2) Constrain residence time estimates for groundwater in deep aquifers of the Mississippi Embayment (3) Delineate extent of mixing from different hydrostratigraphic units along the regional flow path

33. Methods—sampling Water samples collected from 28 municipal wells: –21 in Wilcox aquifer –6 in Claiborne aquifer (overlying) –1 in McNairy/Nacatoch aquifer (underlying) Drill cuttings from three Wilcox wells examined by petrographic microscope (transmitted and reflected light)

36. Compilation of aquifer parameter and well log data Forward and inverse geochemical modeling –evaluate plausible reactions and mixing between sampling points using PHREEQC and NETPATH 2-D numerical modeling of groundwater flow, solute and heat transport using BASIN2 –constrain residence-time estimates and assess leakage Methods—modeling

37. Results Trends along hydraulic gradient: –increases in Wilcox well depth (from 116 to 518 m), T, pH –major-ion facies evolves from Ca-HCO 3 to Na-HCO 3 Ground water anoxic: O 2 0.3 mg/L in 18 wells SO 4 2- decreases and CH 4 increases downgradient Carbonate dissolution/precipitation, cation exchange and redox reactions appear to control chemistry

41. quartz pyrite

42. Results (continued) δD and δ 18 O values fall along regional meteoric water line Offset, gradual enrichments in δD and δ 18 O downgradient Possible explanations for enrichments: (1) proximity to moisture source area (Gulf of Mexico) — leakage of modern meteoric recharge from overlying units — not supported by numerical modeling (residence times ~ 10 6 yr) (2) depletion of isotopic composition of recharge over time —paleorecharge signature preserved downdip by lack of leakage

45. Calculated 36 Cl/Cl ratio × 10 -15 along flow path

46. Comparing High Plains and Wilcox aquifers Differences in recharge –High Plains: locally focused (beneath playas), but regionally diffuse –Wilcox: appears to occur along Coastal Plain margin Common mechanisms of chemical evolution: calcite dissolution, cation exchange, OM oxidation –occur mainly during recharge to High Plains aquifer (oxic) –cation exchange and redox reactions occur within Wilcox (anoxic) Isotopically distinct waters in downgradient portions of both aquifers –upward cross-formational flow into the High Plains aquifer –lack of significant cross-formational flow into the Wilcox aquifer?

47. Take-home messages (1)Combining multiple techniques (hydrochemical analyses, sediment analyses, and flow modeling) gives more complete picture (2)In aquifers being mined (or at risk), need to consider water quality as well as quantity (3)Differences in climate and hydrostratigraphy can override similarities in latitude, physiography, lithology, and land use

48. Acknowledgments Coauthors: B. Mullican, S. Macko, P. Bennett, K. Romanak, J. Banner, R. Brady, R. MorinCoauthors: B. Mullican, S. Macko, P. Bennett, K. Romanak, J. Banner, R. Brady, R. Morin Support: U.S. Department of Energy, USGS, University of Kentucky, GSA, AAPG, GCAGSSupport: U.S. Department of Energy, USGS, University of Kentucky, GSA, AAPG, GCAGS Landowners, agencies, utilities, consultantsLandowners, agencies, utilities, consultants (http://outdoors.webshots.com)