Abandoned wells are a known safety and public health hazard. Their danger received national attention in 1987 when Baby Jessica fell into an abandoned well and the country followed her eventual rescue (Kennedy, 1987), but many years later, children are still falling into wells, a known hazard (Apel, 2015, among others). Aside from the physical hazard of falling into them, abandoned wells can also have a detrimental effect on groundwater quality, such as when surface pollutants enter an aquifer via unfilled abandoned wells (Gass, Lehr, & Heiss, 1977).
Illinois did not begin requiring permits for installation of water wells until the 1960s (Wilson, Rennels, & Roadcap, 2013), so many of the wells in the state are undocumented. The number of abandoned wells in Illinois has been estimated to be in the thousands (Hendrickson, Erickson, & Narve, 1996) and many of these wells were never documented. For example, a well survey in parts of three Illinois counties identified 1,706 total wells. Of these, 788 were not previously documented in the Illinois State Water Survey (ISWS) database (Wilson et al., 2013).
Although this type of well survey is the most accurate method of determining the abundance of undocumented wells, it is both costly and time-consuming to conduct. For this reason, a low-cost technique to estimate the prevalence of undocumented private water wells in a rural setting was developed that relies upon the assumed relationship between the number of structures in an area and the number of wells. This estimation method is not designed to locate individual wells, but to identify areas that are likely to contain undocumented and/or abandoned wells and warrant further investigation. This type of information could be useful to local health departments and/or companies planning to develop rural properties.
A spreadsheet containing all well records of private wells (pumping less than 75 gal/ min) in McDonough County, Illinois, as of September 2015, was provided by the ISWS. The well record information required for this study included the location of the well, date of installation, and the date of sealing if the well was sealed.
Although there was very little documentation regarding the location of private wells in Illinois prior to the 1930s, a survey of private water wells was conducted in 1934 that included 4 of the 16 townships in McDonough County (Illinois State Water Survey [ISWS], 1935). A total of 276 farm or rural wells were identified during the survey. Most of the rural wells (86%) were installed in glacial deposits with depths ranging from 12-90 ft (ISWS, 1935). For the purposes of this study, it was assumed that no wells were missed during the well survey, so the number of wells reported for these four townships was the actual number of wells in 1934. Wells with no installation date were assumed to be older than 1934.
Historical plat books with buildings marked in the rural parts of the county were used to determine the number of structures in the study area. As the plat books do not show individual structures inside city limits, any 1-[mi.sup.2] Public Land Survey System (PLSS) section that contained any portion of the city limits of any town were excluded from the study (Figure 1). Plat books that identified structures were not available for the year of the well survey (1934), so it was necessary to use plat books from 1919 (Howat & Son, 1919) and 1954 (Rockford Map Publishers, 1954) to estimate the number of structures at the time of the well survey. The most recent plat book available for McDonough County that included structures on the map (Rockford Map Publishers, 1997) was used when estimating the likelihood of abandoned wells in the county.
Previous researchers have used aerial photographs, topographic maps, plat maps, or a combination of these resources to identify likely locations of water wells (Blomquist, 1984) or petroleum wells (Aller, 1984; Stout & Sitton, 1984). Our basis for the method used to estimate undocumented wells relies on an assumed ratio between water wells and structures (e.g., houses, barns, churches). As this ratio can change through time as farming practices change (e.g., fewer barns and outbuildings used than in the past), the estimate of undocumented wells was computed for 1934, the time of the aforementioned well survey. Specifically, the four townships within the county that were part of the 1934 well survey were used to establish the ratio of wells to structures in the rural portions of the county at that time.
After scanning the plat maps from 1919 and 1954, GIS software was used to create a shapefile for both years with the locations of each structure marked. The number of structures per 1-[mi.sup.2] PLSS section was determined for the years 1919-1954, and these numbers were used to estimate the number of structures in 1934 through interpolation. PLSS sections were chosen as the base area for computing the well-to-structure ratio because some of the wells in the study...