Groundwater Resources in the Green River Lowlands

Tampico Sand. Overlying the top of the Sankoty Sand is a clay layer that is present throughout most of the region, save for small areas along the Rock and Mississippi Rivers. This clay thickens to the south and east and serves as a barrier (an aquitard) to vertical downward groundwater flow to recharge the Sankoty sand. This clay layer separates the deeper Sankoty Sand from the shallower Tampico Sand. 

The ISWS monitors water levels in the Green River Lowlands. Most wells are still  measured by hand with electric tape  quarterly, although an increasing number are  monitored continuously with submersible pressure transducers , transmitted to ISWS's offices with telemetry apparatus, and displayed on the web in  real-time . Continuous, real time monitoring is critical to understanding the true high- and low-conditions in the aquifer, especially at times of high seasonal withdrawal for irrigation.

Analyzing water levels in wells through time is an invaluable method for determining long term trends and investigating the effect of increased irrigation demands in the area. Note that you may enlarge each of the hydrographs below by selecting them.

In contrast, drawdowns in Whiteside County [WTS-91G shown here] are more consistent year to year. What is striking is that drawdowns in recent, wetter than average years are comparable to extreme droughts of the past. This can be attributed to the fact that irrigated acreage more than tripled since the early 1990's.

In the Green River Lowlands, seasonal changes in the potentiometric surface are more pronounced than in the 1990's. The simplest way to visualize the seasonal effect of irrigation is by subtracting the potentiometric surface during an irrigation season from the average condition; from this, a percent relative difference is calculated by dividing this difference by the average potentiometric surface and multiplying by 100.

The number of irrigated acres has grown steadily from 2012 at a rate of 1-2% per year. The result is that even during average and wetter than average years noticeable declines in available head still occur.

A groundwater flow model of the Green River Lowlands was developed in MODFLOW and calibrated to spring and summertime water levels observed in multiple wells in the year 2020. Drawdown is most prevalent in southeastern Whiteside and northwestern Bureau Counties. Note that observed drawdown from hand measurements in this region is on the order of 25 ft at the center of the cone of depression, but real time data indicates that the actual drawdown at the peak of summer pumping was 40-45 ft. The model, which averages pumping during the summer, cannot at this stage simulate this extent of drawdown and will require further temporal discretization.

In addition to drawdown, the groundwater flow model was used to simulate reductions in natural groundwater discharge when comparing spring to summer flows. The greatest reductions occur in the vicinity of the 20 and 25 foot contours, along the Green River. In the northern portion of the Green River Lowlands, along the Rock River, most of the reductions in groundwater discharging to streams are less than 15%. The actual ecological impacts of these reductions in natural groundwater discharge are unknown, and the model indicates that the streamflow returns to springtime conditions shortly after irrigation ceases.

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.