Kansas Geological Survey, Open-File Rept. 96-1a
Proposed Management Areas--Page 12 of 16
Current rates of development are low but there are local concentrations of high-yielding wells in this management region where the overlying confining units above the Dakota have been removed by erosion. One such area is in southwestern Washington County, where the Dakota has been locally used for irrigated agriculture since the early 1970s (Wade, 1991). Both steady-state and transient flow models were assembled to assess the impact of development on the flow system in this area (Figure 14). The model area boundaries and the dashed line within the model area outline the extent of two topographically-driven local flow systems within the Dakota. The dashed line is a major ground-water divide that separates mostly southerly from mostly northerly flow in the Dakota.
Figure 14. Location of the southwestern Washington Co. modeling study area. The dashed line is a ground-water divide in the Dakota aquifer separating flow to the Republican River from flow to Mill Creek. Observation wells with long-term hydrgraphs are located at A (SW, SW, SW Sec. 14, T. 4 S, R. 2 E.) and B (SW, NE, NW Sec. 1, T. 5 S., R. 2 W.). These hydrographs are shown in Figure 15.
Table 2 shows the water budget for the model area under steady-state conditions. At steady state recharge to the Dakota is estimated to be approximately 0.25 in/yr over the entire model region. Some flow is also contributed to the model area from up gradient sources where the Upper Cretaceous aquitard is present. This is balanced by discharge from the Dakota to Mill Creek along the north boundary of the model and the Republican River along the south boundary.
Table 2. Water budget for the Dakota aquifer in the southwestern Washington County modeling study.
| Sources of Recharge/Discharge |
Pre-development (acre-ft/yr) |
Present development (acre-ft/yr) |
Change | 2X Present development (acre-ft/yr) |
Change |
|---|---|---|---|---|---|
| Recharge | 3,780 | 3,780 | 0% | 3,780 | 0% |
| Upgradient Inflow | 820 | 1,100 | +34% | 1,360 | +66% |
| Total Inflow | 4,700 | 4,890 | +4% | 5,140 | +9.4% |
| Discharge to Mill Creek | 1,670 | 1,630 | -2% | 1,600 | -4% |
| Discharge to Republican River | 3,030 | 2,450 | -19% | 1,930 | -36% |
| Ground-water Pumpage | 0 | 800 | 1,600 | ||
| Total Outflow | 4,700 | 4,890 | +4% | 5,140 | +9.4% |
Most of the high yielding wells in the model area are located in the local flow system that discharges to the Republican River. Mean annual withdrawals from were estimated to be approximately 800 acre-ft/yr from the aquifer based on the records from the Division of Water Resources and estimates of water use for the other wells in the model area. The flow model results indicate a maximum drawdown from present development of approximately 6 ft in T. 5 S., R. 1 E. where most of the irrigation wells are located. The long-term hydrographs of two wells (A and B in Figure 14) support the simulation results and show very little long-term effect from irrigation use (Figure 15). The fluctuations in the early part of the hydrograph for well A show the effects of seasonal withdrawals and recovery in the vicinity of an irrigation well. The later part of the hydrograph of well A and the hydrograph for well B show rising water levels in the late 1980s suggesting increased recharge or decreased pumpage from the aquifer during this period. The summary water budget in Table 2 shows that present development has induced additional flow into the model area from upgradient areas and has reduced discharge to the Republican River and Mill Creek. The water budget indicates that reduced discharge and increased inflow will offset withdrawals if present development is doubled. The total decrease in hydraulic head caused by doubling the existing pumpage is estimated to be approximately 12 ft. The current 0.5 mi spacing between high-yielding wells seems to be appropriate for this part of the management area.
Figure 15. Hydrographs of observation wells in SW, SW, SW Sec. 14, T. 4 S., R. 2 E. (A) and SW, NE, NW Sec 1, T. 5 S., R. 2 W. (B).
Where the Upper Cretaceous aquitard is present in Figure 2, development in this part of the area may induce the flow of better quality water from the outcrop belt into the region where the Upper Cretaceous aquitard is present. Locally, there is a potential for inducing the upward movement of poor quality water from the deeper portions of the aquifer (Figure 3). Because of the potential for inducing the upward flow of poor quality water, the 4 mile spacing is appropriate for high-yielding wells in the Dakota where it is overlain by the Upper Cretaceous aquitard.
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