*3.6. Characterization of Groundwater Atrazine Depletion in Eastern Nebraska Counties*

Given that irrigation and domestic water wells were the deepest wells in the study area and atrazine depletion was observed in deeper wells, we clustered the data based on well depth and re-evaluated the effects of other factors on atrazine depletion. Seven different clusters of counties were observed. While wells in counties of clusters 3, 4, and 5 had low-level atrazine regardless of well depths, wells found in cluster 7 had high atrazine concentration, Figure 7a. To exclude the effect of well depth, additional analysis was performed, comparing two different clusters with similar well depth but different groundwater atrazine concentrations, Table 2. The two clusters eligible for this comparison were cluster 5 (low atrazine) and cluster 7 (high atrazine). Cluster 5 contains wells mostly in groundwater discharge areas, while cluster 7 contains wells predominantly in groundwater recharge areas. Groundwater discharge areas and recharge areas are areas where groundwater flow has an upward and a downward flow component, respectively. The discharge areas of a regional groundwater system are located downstream of a river basin, while the recharge areas of a regional system occupy the upstream water divide of the river basin. For a local groundwater flow system, its discharge areas are at a topographic low, and its recharge areas are at an adjacent topographic high. As shown in the figure below, for cluster 5 counties, most of Burt, Dodge, and Colfax counties are in the downstream areas of Elkhorn River Basin, while Sarpy and Cass counties are in the downstream areas of both Lower Platte and Missouri River Basins.

Similarly, the northwestern part of Lincoln County is in the downstream areas of both North Platte and South Platte River Basins. Although the wells in the eastern part of Lincoln County are located in upstream of the Middle Platte River Basin, they are mainly close to Platte River, which implies that they are in topographic low (discharge areas). For cluster 7 counties, most Polk and all York counties are located in the upstream (recharge areas) of the Big Blue River Basin.


**Table 2.** Comparisons of cluster 5 and cluster 7 characteristics.

The average population that potentially used the groundwater in cluster 5 was three times more than cluster 7. Moreover, wells in cluster 5 received slightly lower precipitation than cluster 7. The average well density per unit land area is low for cluster 5 compared to cluster 7 (Figure 7b–h).

**Figure 7.** *Cont*.

**Figure 7.** *Cont*.

**Figure 7.** Cluster analysis to demonstrate the determinants of low-level groundwater atrazine, Quality Assessed Agrichemical Contaminant Nebraska Groundwater Database, 1995–2014. (**a**) Scatter plot of well depth versus atrazine groundwater concentration. Each color corresponds to a cluster, and a box marks each cluster's prototypical data point (exemplar) while all cluster members are joined to their exemplars with lines. (**b**) Atrazine level by counties (**c**) Average precipitation by counties (**d**) Average temperature by counties (**e**) Population by counties (2018) (**f**) Changes in the groundwater level of counties of observed clusters (**g**) River basins of counties of observed clusters (**h**) Density of wells in the counties of observed clusters. Counties with utilization wells were labeled "U", and counties with monitoring wells were labeled "M".
