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County Well Index (CWI) location-verified and interpreted well records were summarized by PLS section. Results show the general distribution of aquifer use in the state. Classification is based on the aquifer code with the greatest frequency in each section; in cases where multiple aquifer codes have the same count, classification between those codes is arbitrary:
• Surficial sand and gravel – aquifer close to the land surface. Like a sandbox, vulnerable to contamination due to lack of overlying fine-grained material; recharges quickly
• Buried sand and gravel – aquifer not close to the land surface, less vulnerable to contamination due to presence of overlying – fine-grained material; recharges slowly
• Sandstone bedrock – aquifer typically not close to the land surface, contamination vulnerability depends on composition and thickness of overlying material
• Carbonate bedrock (limestone) – aquifer often close to the land surface, contamination vulnerability depends on composition and thickness overlying material. Because carbonate bedrock dissolves over time, water can move quickly over long distances through conduits and caves and is highly vulnerable to contamination in near-surface conditions.
o see areas mapped as karst on GRAPS Pollution Sensitivity of Near-Surface Materials
• Crystalline bedrock – aquifer often close to the land surface, contamination vulnerability depends on overlying material. The rock itself is relatively impermeable, so water moves through fractures only; wells can have low yields and be vulnerable to contamination.
o See areas in northeastern and southwestern Minnesota mapped as bedrock at or near surface on GRAPS Pollution Sensitivity of Near-Surface Materials
• Weathered material – aquifer typically not close the land surface, less vulnerable to contamination due to presence of overlying fine-grained material. Water quality can be highly mineralized; used as an aquifer because of no overlying sand and gravel; recharges slowly
Data source: Not available for download |