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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole <br />hazard assessment in 2020. <br />4.1.2.10. Detection & Warning <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored sinkhole <br />hazard assessment. <br />4.1.2.11. Critical Values and Thresholds <br />4.1.2.11.1. Bedrock material: Areas susceptible to sinkholes (karst terrains) are underlain by <br />water-soluble, but relatively impermeable bedrock such as limestone (calcium carbonate). <br />Soluble rocks dissolve when exposed to certain acids, including acidic water. Over time, acidic <br />water flowing through joints and cracks will dissolve and remove large amounts of soluble rock <br />creating many void spaces. In more unusual instances, sandstones or even quartzite may develop <br />sinkholes. In these cases, the bedrock is more permeable, but less soluble. Slower sinkhole <br />development may occur in these rocks. <br />4.1.2.11.2. Water acidity: Acidic surface water and groundwater is required for natural sinkhole <br />formation as the agent that dissolves soluble bedrock. Pure water has a pH of 7.0, which is neutral <br />— neither acidic nor base. However, water in nature is not pure. Instead, it contains natural <br />impurities which make it acidic. Unpolluted rainwater has a pH of around 5.6 (acidic). Rainwater <br />in Minnesota contains atmospheric pollutants which further lower the pH, increasing acidity. <br />Once at the surface, water can become further acidified by exposure to nitrogen fertilizers or <br />other chemicals. When this water infiltrates into the bedrock it begins to gradually dissolve any <br />carbonate rocks. <br />4.1.2.11.3. Bedrock depth: For a void to cause a collapse of the overlying surface material it must <br />be close to the surface. Active karst areas have carbonate bedrock less than 50 feet below the <br />surface. Transitional karst areas have carbonate bedrock covered by material between 50 and <br />100 feet. In some instances, sinkholes can occur in these conditions as well. Covered karst areas <br />have more than 100 feet of overburden. Sinkholes are unlikely to develop in such deep <br />conditions. <br />4.1.2.11.4. Bedrock topography. Once water penetrates the soil, it will arrive at the bedrock layer. <br />Typically, the bedrock is much less permeable than the overlying unconsolidated soils which <br />promotes lateral water flow. The water will flow according to the topography of the bedrock <br />finding crevices and valleys that collect water until a penetration point can be found into the <br />bedrock. <br />4.1.2.11.5. Joints, fractures, and bedding planes: These features provide easy routes for water to <br />travel through the rock. As water moves through this network of joints, fractures and bedding <br />planes, chemical action of the acidic water dissolves the bedrock. Joints and fractures are often <br />oriented in parallel and perpendicular patterns. Because of this, voids and sinkholes also are often <br />aligned to follow these patterns. <br />4.1.2.11.6. Water table: Fluctuations in ground water levels can affect sinkhole activity. Abrupt <br />21 <br />