05/15/2025
🚰 Why Some Public Supply Wells Fail Water Quality Standards – Even When the Pilot Hole Looked Good
Many public supply wells are built based on the water sampling taken during pilot hole drilling. The problem? These samples often come from the highest-yield, easiest-to-access zones – and don't provide a full profile of the aquifer.
Here's what BESST, Inc. found in a study of 143 municipal supply wells in California and Nevada:
🔍 Low-permeability layers (like clays and silts), are usually skipped during testing, but these are often the source of elevated levels of arsenic, iron, manganese, and nitrate. These zones don’t contribute much water, so they’re often ignored — but chemically, they matter a lot.
💡 Key Insight: In wells that failed to meet standards, BESST found that up to 71% of the problem metals came from these “ignored” fine-grained zones.
Why this happens: These fine layers release metals through a process called reductive dissolution. When we pump the surrounding zones, we pull more of these contaminants into the well.
📌 Recommendation: Instead of testing in high-yield zones, use a long-screened test well (LSTW) that samples across the full vertical profile — especially near clay or silt boundaries. This will give you a more complete picture of water quality before you invest in a full well build.
Better testing = better wells = safer water. 💧
Contact us to get more information and let us show you a better way to sample your pilot test well.
