Enhancements made to groundwater levels viewer

Knowing just how high groundwater will rise is critical information for anyone doing work beneath the surface on Cape Cod. Septic installers, engineers designing stormwater and wastewater infrastructure, and those installing underground utilities all need to understand whether the infrastructure they place below ground will remain dry under the wettest conditions.

In the past, estimating seasonal high groundwater levels was a cumbersome process. Practitioners relied on hard-to-read paper maps and worksheets, making what should be a routine calculation both time-consuming and prone to error.

In 2022, the Cape Cod Commission debuted the High Groundwater Levels Data Viewer, providing a more accessible, up-to-date way to estimate seasonal high groundwater levels anywhere on Cape Cod. Recently refreshed with visual and usability updates, the Viewer continues to serve as an essential resource for engineers, surveyors, septic professionals, and local officials.

How the Viewer Works

Groundwater levels fluctuate seasonally across the Cape. Levels are typically highest in mid- to late spring, lowest during the summer, and then rise again through the fall and winter. Because of this variability, a single groundwater measurement taken in the field does not, on its own, represent the highest level groundwater may reach.

The High Groundwater Levels Data Viewer allows users to bridge that gap. A practitioner digs a hole to take an on-site measurement of groundwater depth, then uses the Viewer to identify the applicable index well and groundwater adjustment for that location. By applying this adjustment to the measured depth, users can estimate the seasonal high groundwater level for a given location at any time of year. 

When a user clicks on a project location in the Viewer, a pop-up displays the relevant index well, the most recent groundwater measurement, and the adjustment needed to estimate seasonal high groundwater. This information feeds directly into septic system design, stormwater calculations, and other permitting and construction decisions.

Groundwater Monitoring Across Cape Cod

Although the Cape Cod Aquifer is classified as a sole-source aquifer, it is made up of six distinct groundwater lenses. Combined with Cape Cod’s variable precipitation patterns, this means groundwater levels can differ significantly from one area to another, and from one month or year to the next.

To track these changes, Cape Cod Commission staff, in partnership with the U.S. Geological Survey (USGS) and Massachusetts Department of Environmental Protection, collect monthly groundwater measurements at 26 monitoring wells across the Cape. Measuring the same wells every month provides a clear, real-world picture of how groundwater responds to seasonal conditions and long-term trends. At several locations, the USGS has also installed automated data loggers to supplement manual measurements.

These monthly measurements are published by the Commission as a service to town officials, engineers, and other interested users. Records for select wells extend back to the 1950s and are available through the USGS Water Data for the Nation at USGS | National Water Dashboard.

Why High Groundwater Matters

High groundwater levels are a major contributor to septic system failures, wet basements, and other infrastructure problems. Under Massachusetts Title 5 and local regulations, septic systems, stormwater structures, and other below-ground infrastructure must maintain adequate separation from groundwater, even under the wettest seasonal conditions.

Historically, some permitting authorities required contractors to wait until spring, when groundwater levels were naturally highest, to document high groundwater for septic system approvals. To address the need for year-round estimation and account for annual variability, the Cape Cod Commission developed Technical Bulletin 92-001, Estimation of High Groundwater Levels for Construction and Land Use Planning.

That methodology, updated over time to reflect new data, replacement wells, and improved monitoring, now lives in a far more accessible form through the High Groundwater Levels Data Viewer.

To learn more about estimating seasonal high groundwater levels and to access current and historical data, visit the Cape Cod Commission’s Estimating High Groundwater webpage and explore the High Groundwater Levels Data Viewer.