Water Quality of the Rock River Water Supply Planning Region

Groundwater quality is an important factor for a given water source's suitability for public water supply. Water quality can be characterized and assessed in a number of ways that can be important for public water supply. The discussion here focuses on contaminants, including both human-derived contaminants and natural contaminants. The human-derived contaminants investigated here in shallow aquifers (<300 ft deep) include nitrate-nitrogen, chloride, and total dissolved solids (TDS). Natural contaminants in Illinois most notably include arsenic, radium, chloride, and fluoride.

Nitrate is the most common human derived groundwater contaminant in the world. It has numerous sources, including agricultural activities (synthetic fertilizer, livestock manure, soil disruption) and human waste (sewage and septic systems). While the primary drinking water standard for nitrate as nitrogen is 10 milligrams per liter (mg/L), concentrations greater than 2 to 3 mg/L generally indicate contamination from human sources ( Panno, Kelly et al 2006 ).

The assessment of Nitrate was limited to shallow wells (<300 feet deep). Elevated concentrations of nitrate were found in much of the region, especially in the northern half, as well as along rivers such as the Rock, Pecatonica, and Mississippi (see interactive map below). The majority of wells sampled in sand and gravel aquifers, especially in the Green River lowlands and the Rockford region, had elevated nitrate concentrations. Shallow bedrock wells in the northern half of the region (Jo Daviess, Stephenson, Carroll, Ogle Counties) also commonly had elevated nitrate levels. Nitrate concentrations tended to be relatively low in the southernmost part of the region.

Chloride is a common contaminant and when it is found in shallow aquifers, it generally indicates human activities; though natural sources exist as well. Where there are no significant natural sources, concentrations greater than 10 to 15 mg/L generally indicate human contamination ( Hwang et al. 2015 ;  Panno, Hackley et al., 2006 ) . In the Rock River region, chloride concentrations were most likely to be elevated in Rockford, likely due to winter rock-salt (NaCl) runoff. Elevated levels in other, more rural parts of the region are probably the result of agricultural runoff and/or septic/sewage discharge, as well as road salt runoff.

Well depth is an important variable for nitrate and chloride. The sources for both nitrate and chloride contamination are found at or near the land surface, so it is not surprising that shallower wells, especially those less than 100 feet deep, tend to have higher concentrations of these contaminants.

There are many contaminants found in groundwater that can occur naturally, and some of these can be health concerns and have primary drinking water standards; in Illinois, most notably arsenic, fluoride, and radium. Others are mainly nuisances, including iron, manganese, hardness (calcium + magnesium), and total dissolved solids (TDS).

Arsenic is a common contaminant in sand and gravel aquifers throughout the state, although typically not found in bedrock aquifers. The assessment for Arsenic is limited to shallow wells (<300 feet deep). Arsenic concentrations greater than the primary drinking water standard (10 micrograms per liter (μg/L)) are common in sand and gravel aquifers in the southern half of Rock River region, especially in Bureau County. As expected, with a few exceptions, elevated arsenic levels are generally not found in shallow bedrock aquifers. There is widespread geographic variability in arsenic concentrations in the region, which has also been observed in other parts of the state. Well depth is not an important variable affecting arsenic concentrations.

Community water supplies that rely on the St. Peter and Ironton-Galesville sandstones often have to deal with elevated levels of radium, which is produced naturally within the aquifers. The primary standard for total radium (the sum of the isotopes 226Ra and 228Ra) is 5 picocuries per liter (pCi/L). Of the 59 wells sampled for radium by IEPA that are open to the St. Peter and/or Ironton-Galesville aquifers, 12 (~20 percent) had levels greater than the drinking water standard. The use of these bedrock aquifers as drinking water sources requires treatment in order to meet the drinking water quality regulations. Domestic wells are not routinely sampled for radium analysis, but it is likely that if they are open to the St. Peter sandstone, such as in Ogle County, they may have levels of concern.

In most of Illinois, the bedrock aquifers are part of the Illinois Basin, which deepens from north to south. In the northern part of the Rock River region, the Cambrian-Ordovician aquifers (Galena-Platteville, St. Peter, and Ironton-Galesville) are at or relatively close to land surface. Because these aquifers deepen to the south of the region, wells must be drilled deeper and natural TDS and chloride levels increase ( Panno et al, 2002 ). A map of chloride concentrations in wells greater than 300 feet deep shows that, in most of the region, chloride concentrations are fairly low, generally less than 50 mg/L. In the extreme south in Bureau and Henry Counties, however, several wells have chloride concentrations that exceed the secondary drinking water standard of 250 mg/L.

Because of the higher TDS levels in the southern part of the region, many elements and aqueous species also have elevated concentrations, including fluoride, which has both a primary (4 mg/L) and secondary (2 mg/L) drinking water standard. The assessment for Fluoride was limited to deeper wells (>300 feet deep). Like TDS and chloride, fluoride tends to be elevated in Bureau and Henry Counties, and into Rock Island County.

Nuisance contaminants are termed as such because they may induce scale build up and affect aesthetic qualities such as taste. Shallow groundwater in the Rock River region tends to be hard to very hard, and have elevated iron concentrations commonly greater than 1 mg/L. Ammonium levels also tend to be high in the southern part of the region, especially in wells deeper than 300 feet and particularly in Rock Island County. These contaminants can be dealt with using conventional treatment technologies.

However, in the Driftless portion of Illinois, an area in the northwest corner of the Rock River Region, the geology is different from the rest of the region and thus so is the water quality.

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.