1. SMALL-SCALE SPATIAL VARIATION OF WATER QUALITY WITHIN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER ARKANSAS, USA Ken Steele 1, Tim Kresse 2 and Ralph Davis 1 1 Department of Geosciences, University of Arkansas, Fayetteville 2 Water Division, Arkansas Dep.t of Environmental Quality, Little Rock

2. OBJECTIVE To ascertain spatial variability of major ion concentrations on small scale within the alluvial aquifer in Arkansas

3. Pulaski County ARKANSAS County Woodruff County Monroe Alluvial Aquifer U S A

4. Alluvial Aquifer Composition Sand, silt, clay grades downward to sand and gravel (~ 45 m thick) Discontinuous lenses of clay, silt or sandy silt Overlying unit (6 m thick) composed of fine silt and clay Mineralogical heterogeneity Rainfall 1270 mm annually but little recharge during summer Yield3,785 to 11,355 liters per minute

5. Site DescriptionsSiteCrops Soil Mapping Unit Reliefm Well Depths m Water Depth m PulaskiCounty Soybean Vegetables Keo & Rilla – well drained Perry – clayey ~ 0.45 Shallow 9 - 10 Deep 24 – 30 Shallow 7-9 Deep ~ 8 MonroeCounty Soybean Rice Foley & Dundee- poorly drained ~ 0.3Shallow 8 - 9 Deep 30 - 34 Shallow 4 - 5 Deep ~ 5 WoodruffCounty Soybean Dubbs – permeable Amagon-more clay ~ 0.3 Shallow 9 -15 Deep 24 - 30 Shallow ~ 6

7. Monroe County 3 shallow monitoring wells 3 deep irrigation wells Pulaski County 3 shallow monitoring wells 1 domestic well 3 deep irrigation wells Monitoring wells 20 - 212 m apart Irrigation wells 1.3 km apart Irrigation wells 0.8 km from monitoring wells Woodruff County 4 monitoring wells 1 domestic well

8. Sample Collection Site Shallow monitoring and domestic wells Deep irrigation wells Monroe Pulaski counties June 24-25 2002 August 11-12, 2002 August 11-12 2002 Woodruff County June 18 1996 None

9. Monroe County Shallow Monitoring wells ~ 30 feet deep Deep Wells ~ 100 feet Smallscale 0500 Feet

10. Septic tank PulaskiCounty

11. MonroeCounty Monitoring Well

12. Monroe County Pig pen

13. 14.45 Monroe County High SO 4, Cl, and NO 3

14. Temporal Spatial Variation Plots of recharge versus non-recharge periods ion concentrations allow simultaneous comparison of spatial and temporal variations

15. Recharge Conditions Wells assigned recharge or non-recharge conditions based on: Typical recharge period (October – June)  Septic tank effluent and Irrigation return flow recharge not likely Best scientific judgment based on:  Rainfall  Ground water levels

16. max/min max/min M P W M P W Shallow 1.9 1.6 4.1 Deep 1.9 1.7 max/min max/min M P W M P W Shallow 3.7 4.9 4.4 Deep 1.4 4.7 max/min max/min M P W M P W Shallow 3.7 4.9 4.4 Deep 1.4 4.7

17. max/min max/min M P W M P W Shallow 5.3 1.8 3.5 Deep 2.0 -- max/min max/min M P W M P W Shallow 2.9 1.6 4.5 Deep 2.0 -- max/min max/min M P W M P W Shallow 2.9 1.7 >6.7 Deep 1.5 1.3 max/min max/min M P W M P W Shallow -- 5.4 -- Deep -- -- --

19. Calcite-richunit Clay-rich Clay-rich units Clay-richunit Non-recharge period water table Recharge tableRecharge table Sandy silt Wells

20. CONCLUSIONS There is prevalent and extensive small- scale spatial variation of ion concentrationsThere is prevalent and extensive small- scale spatial variation of ion concentrations There is temporal variation for Ca, Mg, Na, SO 4, HCO 3There is temporal variation for Ca, Mg, Na, SO 4, HCO 3 Significant cation exchange occurs for Ca and Na (and some for Mg and K)Significant cation exchange occurs for Ca and Na (and some for Mg and K) Deep wells may or may not have ion concentrations greater than shallow wellsDeep wells may or may not have ion concentrations greater than shallow wells

21. CONCLUSIONS (continued) Spatial variation of water quality is attributed to: Spatial variation of water quality is attributed to: – discontinuous sand, silt and clay lenses with heterogeneous mineralogy – differences in residence time – lower water tables can allow fine-grained sediment units to de-water thus increasing ion concentrations Temporal variation is caused by: Temporal variation is caused by: – fluctuating water tables that can change flow paths

22. SUMMARY Complex physical and mineralogical heterogeneity of the alluvial aquifer sediments and varying recharge cause significant variability of the ground-water chemistry

23. max/min max/min M P W M P W Shallow 6.8 3.6 4.3 Deep 1.3 --

24. max/min max/min M P W M P W Shallow 3.7 4.9 4.4 Deep 1.4 4.7

25. max/min max/min M P W M P W Shallow 1.9 1.6 4.1 Deep 1.9 1.7

26. max/min max/min M P W M P W Shallow 2.9 1.6 4.5 Deep 2.0 --

27. max/min max/min M P W M P W Shallow 5.3 1.8 3.5 Deep 2.0 --

28. max/min max/min M P W M P W Shallow 2.9 1.7 >6.7 Deep 1.5 1.3

29. max/min max/min M P W M P W Shallow --5.4 -- Deep -- -- --