Steve Mabee, geologist

Steve Mabee, PhD

Massachusetts State Geologist| UMass Amherst faculty

It has been difficult to get recent soil and groundwater maps for the Boston metropolitan area. What can you tell us about the typical conditions in this part of Massachusetts?

Massachusetts used to be glaciated and when the glacieers retreated they left depositions like the drumlin that is now Beacon Hill. What’s interesting about the Boston area is that, post-glaciation, it was inundated by the ocean. When the ocean retreated, it left Boston Blue clay. Clear of the ocean, the coast became very marshy and so a typical cross section in the Boston area is bedrock/ glacial till/Boston Blue clay/marsh muck/ fill. Glaciers shaped the groundwater landscape. Glacial valleys provide very linear aquifers, often don’t align with modern rivers. A lot of glacial lakes were dammed by sediments, varves, and lacusterine deposits. After they drained, sand and gravel was left on top, leaving us with present-day surficial aquifers that are mostly unconfined.

Most of Boston was built on fill material which can be unstable. But, why is it unstable? Do you know of any geological impacts this fill has had?

Fill is unstable because it is atop old, uncompacted marsh deposits that are also above Boston Blue clay which is susceptible to liquefaction.

Geology is often seen as very static but a small shift can mean big changes for people. In the next 50 to 100 years, do you see any problems arising from ignoring geology?

If a building is on till or bedrock they should be stable. But areas at risk of saltwater intrusion could face a lot of wetting and drying of pilings and that really makes them deteriorate.

We’ve noticed that many geological datasets have a very geometric cutout where Boston and its surrounding towns are. Why is that?

Expense. It’s expensive to conduct studies in densely populated areas. Data provided by well drillers is available online but only 15% of well locations have gps locations - not in as good shape as other states.

Chris Hirsch

Environmental Scientist| Neponset River Watershed Association

How and when do you think of soils?

The only time we think about soil and sediment control is during construction. We are very strict with the bylaws and also do a fair amount of green projects.

We do soil assessments for the site, determining infiltration rates and degree of connectivity to groundwater (gw). If a site is highly interactive with the water table then we do something different and line the project or ensure that site drainage would not impact the local groundwater levels. We generally do these projects using the 319 grant system, a federal program established through the Clean Water Act and administered by the state. Work done with 319 money has to be done on public lands and so do much of our work with parks and schools and include an educational component.

So, you don’t work with private properties?

I don’t typically interact with private property but we are asking the town to account for all bmps. There are a lot of methods and devices to deal with stormwater runoff. They have pollution plans set by the epa that outline water quality standards. It’s in the best interest of the town.

bmp stands for Best Management Practices, correct?

Yeah, when we say bmps that includes the technology and techniques used to manage stormwater runoff. So, a lot of developers are implementing Best Management Practices but there’s no cumulative record that people can reference to see what works best where. Also, it would help the town to know what their landscape looks like.

Stormwater management plays a big role in maintaining groundwater levels but we’re seeing water tables drop in places like Boston. Is that an issue here? What factors do you see impacting this? Is infiltration the only remedy?

Yes, decreasing groundwater levels is definitely an issue here. Our primary concern is the infiltration of groundwater into sewage lines by way of cracks. So, the town ends up paying Deer Island by the gallon to treat groundwater while also lowering the water table. We do an educational program with schools. We have done a rebate program for high efficiency showers, toilets, and other fixtures through the mwra. We try to run campaigns- Brown is the New Green, where we encourage homeowners to decrease lawn watering. We try to look at enforcing private wells but the town doesn’t like to superimpose on private land. Each town has their own response to drought but we’d like to see a coordinated statewide response. If one town does one things and another does their own thing then the town downstream suffers from the practices of the town upstream. Unification of efforts would respond more effectively to hydrological considerations.

What is the reason for the drop in groundwater levels? A recent report indicated that groundwater levels have been on the rise in New England over the last hundred years.

The scenario has flipped. The main issue is basically continually pulling water from the ground then