Fifteen months after all homes in the Little Pond area of Falmouth were connected to the sewer system, sampling has shown that groundwater on the Maravista peninsula has decreased levels of nitrates.
Timothy D. McCobb presented the positive project update to the Falmouth Water Quality Management Committee on Monday, September 20.
The project, which has been ongoing for the past six years, has funding for one more year of data collection.
Mr. McCobb, a hydrologist with the USGS New England Water Science Center, has been working on the groundwater study since its inception in 2016. The study covers an area of around 225 homes between Cedar Street and Hiawatha Street.
While USGS is still awaiting the results from September 2021, September 2020 testing showed significant changes in nitrate when compared to prior testing, Mr. McCobb said. Samples indicate that nitrate levels are decreasing in the groundwater.
The study is focused on how long after sewering it takes for nutrient loads to decrease, which in the Little Pond sewer area is 15 months out from full connections, Mr. McCobb said.
“Our project objective has been to evaluate the effects of replacing the septic systems with sewers on water quality in one of these density-developed coastal neighborhoods and to try to get at questions like, what does the special and temporal distribution look like below these neighborhoods?” Mr. McCobb said.
In May 2016, 18 monitoring wells and 14 multilevel samplers were installed to monitor water levels and the quality of groundwater in the study area before and after sewering. Sewers were installed on 1,350 properties around Little Pond, opening the connection to the new lines in 2017.
There is also a control site located on Seacoast Shores.
The work is being conducted in collaboration with the Environmental Protection Agency—Region 1 and the EPA—Southeast New England Coastal Watershed Restoration Program.
Quarterly testing at a site on Reynolds Street indicated a decrease in nitrate from maximum levels of 18 to 20 milligrams per liter, Mr. McCobb said. June 2021 numbers are showing in the 14 to 16 range. Over time the median milligrams per liter of nitrate has dropped as low as five, Mr. McCobb said.
“There are questions of variability and maybe seasonality in that signal, but you do see over time that that range is getting smaller,” Mr. McCobb said.
The study also tracked the rate of groundwater flow to the coastal pond. For the Maravista area, the mean groundwater travel time was 2.5 years, with a maximum of 10 years in areas farther from the shoreline, Mr. McCobb said.
Mr. McCobb said there is a groundwater divide running west of Maravista Avenue through the study area. Around 35 to 40 percent of the area is flowing into Little Pond. The remaining groundwater flows either south or east to Great Pond, Mr. McCobb said.
“Based on water use in this area, septic return flow represented about 20 to 25 percent of the natural recharge there. I just wanted to bring that up as well, the recharge has changed,” Mr. McCobb said.
Sampling is also taking place in conjunction with the EPA, which is looking at pharmaceuticals, personal care products, PFAS, hormones and bacteria, Mr. McCobb said. From Mr. McCobb’s understanding, finding a trend for these materials is difficult because it is dependent on people’s use.
The study is also measuring sucralose as an indicator of wastewater. The most recent data from June 2021 showed a significant drop in the amount of sucralose, or artificial sweetener, in the groundwater, Mr. McCobb said.
The data do not trend down linearly, however; measurements from April 2018 show a significant increase from prior sampling years. Mr. McCobb believes this is because numbers are dependent on consumer use.