Groundwater Quality in the Ogallala Aquifer Region in Texas

1. Groundwater Quality in the Ogallala Aquifer Region in Texas Srinivasulu Ale Assistant Professor (Geospatial Hydrology) Sriroop Chaudhuri Postdoctoral Research Associate Texas A&M AgriLife Research at Vernon, TX

2. Presentation Outline • Groundwater quality monitoring in Texas. • Spatio-temporal variability of SO4, Cl, NO3 and TDS (salinity) concentrations in groundwater in the Ogallala aquifer region. • Hydrochemical facies composition and major geochemical types of groundwater in the Ogallala aquifer region in Texas. • Potential future directions.

3. Texas Aquifers and Groundwater Use • About 60% of the total water used is supplied from groundwater. • Greater than 75% of the extracted groundwater is used for irrigation. • More than 95% of rural households rely on groundwater.

4. Groundwater Quality Monitoring in Texas • Texas Water Development Board (TWDB) maintains a groundwater quality database with data from 1896. • More than 120,000 groundwater wells and spring locations monitored. • Each aquifer is sampled once every 4 to 5 years and about 1000 water quality samples are collected annually. • 33 different water quality parameters are measured.

5. Well Depth and Water Use Classes • Geographic segregation in well depth: shallower wells in the southern parts of the Ogallala region. • Irrigation wells dominate the region, followed by domestic wells.

6. Groundwater Quality Data Processing • SO4, Cl, NO3and TDS data for the period from 1960 to 2010 were obtained from the TWDB data base. • Quality Control Tests: • Reliability code provided by the TWDB. • We conducted ion charge balance analysis. • Data was aggregated over decadal scale: • 1960s (1960-69), 1970s (1970-79)….2000s (2000-2010) • Most recent observation for each well was included in the analysis, if it has multiple readings in any decade. • Data was compiled and mapped in ArcGIS.

7. Groundwater Quality Thresholds Source: USEPA, USDA

8. Spatio-temporal Variability of Sulfate (SO4) Ogallala Seymour & Blaine Pecos Valley Trinity Edwards-Trinity • Clustering of higher values (>SMCL) in the southern Ogallala in all decades. • Rustler, Castile and Salado evaporite Formations in the southern Ogallala release SO4 and Cl upon dissolution. • Recent oil exploration activities in the Permian basin.

9. Sulfate by Well Depth

10. Spatio-temporal Variability of Chloride (Cl) Ogallala Seymour & Blaine Pecos Valley Edwards-Trinity Gulf Coast • Clustering of higher values (>SMCL) in the southern Ogallala in all decades. • Aquifer mineralogy. • Oil exploration activities. • Salt water intrusion in the Gulf Coast aquifer.

11. Chloride by Well Depth

12. Spatio-temporal Variability of TDS (Salinity) Ogallala Seymour & Blaine Pecos Valley Trinity Edwards-Trinity Gulf Coast • Clustering of higher values (>SMCL) in the southern Ogallala in all decades. • Aquifer mineralogy. • Solute exchange due to pumping • Climate – high ET. • High salinization in Seymour, Trinity, Gulf Coast aquifers.

13. TDS by Well Depth SMCL • Median TDS exceeded SMCL in shallow wells since the 1960s, indicating persistent groundwater salinity problems in this region. • Observations in > 50% wells exceeded the SMCL since the 1960s.

14. Hydrochemical Facies – Piper Plots • Indicate major groundwater geochemical types. • Indicate regional differences in groundwater geochemistry. A: Ca + Mg >>Na + K (hard groundwater) (Alkali metals) >> (Alkaline earth metals) B: Na + K >>Ca+ Mg (soft groundwater) C: SO4+ Cl>>HCO3 (strong acids) >> (weak acids) D: HCO3>>SO4+ Cl Hydrochemical Facies 1 : Ca-Cl-SO4 2 : Ca-HCO3-Cl 3 : Ca-Na-Cl-SO4 4 : Ca-Mg-HCO3(Fresh recharge) 5 : Ca-Na-HCO3-Cl 6 : Na-Cl 7 : Ca-Na-HCO3 8 : Na-HCO3-Cl 9 : Na-HCO3

15. Hydrochemical Facies: TDS Observations in 2000s SO4-Cl facies Transition from HCO3 to SO4-Cl facies HCO3 dominant facies

16. Hydrochemical Facies: (SO4+Cl)/HCO3 • Clear geographic segregation of high and low (SO4+Cl):HCO3 ratios. • Is there a relationship between (SO4+Cl):HCO3 ratio and salinity (TDS)? HCO3 dominant facies SO4-Cl facies

17. (SO4+Cl)/HCO3 and Salinity Interactions • Linear relationship at higher salinity (TDS >500 mg/L). • More influenced by HCO3 at low salinity (TDS <500 mg/L) and follows an exponential logarithmic trend. • SO4and Clare main agents causing extensive groundwater salinization. All TDS observations

18. Effect of Depth to Water Level on TDS • Increasing groundwater salinization with shallower water levels, found mostly in the southern Ogallala. • This is contrary to general trends in other aquifers (ex: Trinity aquifer). • Solute concentrations vary inversely with depth to water level. • Depth attributes have nominal effect on HCO3 levels.

19. Spatio-temporal Variability of Nitrate Ogallala Seymour Pecos Valley Edwards- Trinity Gulf Coast • Clustering of high values (>MCL) in the southern Ogallala in all decades. • Found a close association between agricultural activities and NO3 concentrations in the Rolling Plains. • High nitrate levels in south Texas (Gulf Coast aquifer) – agriculture and irrigation return flow

20. Nitrate by Well Depth and Water Use Class Well Depth Water Use Class

21. Hydrochemical Facies: NO3 Observations • Clear geographic segregation of high NO3 concentrations. • High NO3concentrations clustered in the SO4 and Cl facies, probably indicating a common source

22. Nitratevs. Other Water Quality Parameters • Significant positive correlation with Ca, Mg, Na, K, SO4, Cl and TDS. • Is there a common source? • Significant influence of NO3 on groundwater salinization. • Significant positive correlation with SAR (Sodium Absorption Ratio), which is a major concern for irrigation water quality. • Why Spearman Rho changed over time?

23. SUMMARY • Salinity is a major threat to water quality since the 1960s and it is increasing over time, in the Ogallala aquifer region in Texas. • Groundwater salinity (TDS) is largely caused by SO4 and Cl. • Shallow wells are more prone to contamination. • Domestic wells are more at risk than public supply wells. • NO3 concentration is substantially high in the southern Ogallala region, and it is increasing since the 1960s. • Hydrochemical facies change from the north to south of the Ogallala aquifer region in Texas.

24. Potential Future Directions • Do hydrochemical facies further change with space due to: • Changes in sedimentary depositional conditions in the Ogallala aquifer? • Regional groundwater flow paths? • What are the prime sources of SO4 and Cl? • What kind of anthropogenic activities, other than agricultural, affect water quality in the Ogallala region (hydrocarbon exploration?) • Is detailed well-by-well assessment necessary? • How does climate affect water quality? • What other water quality parameters need attention?

25. Thank You Contact details: Srinivasulu Ale Assistant Professor (Geospatial Hydrology) Texas A&M AgriLife Research and Extension Center Vernon, TX 76385 Email: sriniale@ag.tamu.edu Phone: 940-552-9941 x 232

26. SUMMARY • Salinity is a major threat to water quality since the 1960s and it is increasing over time, in the Ogallala aquifer region in Texas. • Groundwater salinity (TDS) is largely caused by SO4 and Cl. • Shallow wells are more prone to contamination. • Domestic wells are more at risk than public supply wells. • NO3 concentration is substantially high in the southern Ogallala region, and it is increasing since the 1960s. • Hydrochemical facies change from the north to south of the Ogallala aquifer region in Texas.