1995 letter to Falmouth, MA Wastewater Facilities Manager
[Note: This letter was part of a Buzzards Bay NEP effort in the mid to late 1990s to raise awareness of Town of Falmouth residents and municipal officials about the eutrophication of West Falmouth Harbor, and the role of the town’s sewage treatment facility and landfill as contributing nitrogen sources. Our efforts led to other studies by the Massachusetts DEP and the Cape Cod Commission resulting in new requirements for advanced nitrogen removal at the town’s wastewater facility. The town’s advanced nitrogen removing wastewater facility came on line in the fall of 2005. It will take 5 to 10 years for the benefits of this facility to be felt in the harbor, due to the transit time of groundwater. In 2006, Massachusetts DEP approved a nitrogen TMDL for the West Falmouth Harbor Watershed (available at the oceanscience.net website). Because the wastewater facility is already using best available technology to reduce nitrogen additional measures to meet the TMDL will eventually be needed including sewering additional homes in the watershed.]
August 28, 1995
Falmouth, MA 02540
Based on our telephone discussion a couple of weeks back, you raised some questions as to how the Buzzards Bay NEP calculated nitrogen loading from the Falmouth Sewage Treatment Plant that we reported in our Progress Report to the selectmen dated July 23, 1995. In this report we estimated nitrogen from the plant to be 11,800 kg year, but the report did not explain how this estimate was derived.
Explanation of loading estimate:
Because there are many information gaps regarding loading from the plant, we made the following key assumptions.
1) Nitrogen removal in the spray irrigation area was better than nitrogen removal in the effluent infiltration basin area.
2) Wells #2 and 2A (see attached map) are probably not in the direct path of the plume from the groundwater infiltration basins. If the observed nitrogen concentrations in these wells represent somewhat lower concentrations in the periphery of the plume, then the observed groundwater nitrogen concentrations are inadequate to define loading from the infiltration basins.
3) Any calculation of mass loading from the spray irrigation area that is based on groundwater concentrations under the spray irrigation area must take into account the volume of recharge of both the spray effluent and rainfallat the site. Additional assumptions relating to this estimate are as follows:
a) Groundwater concentrations of dissolved inorganic nitrogen under the spray irrigation area (wells 15, 16, and 17) have typically ranged 6-12 ppm since 1991, with peaks (presumably during high spray irrigation use) observed of 10-12 ppm. For our calculations, we conservatively used 12 ppm DIN.
b) Rainfall in the area averages 42″ (1.07 m) per year, with a 50% typical recharge. In light of the spraying of effluent at the site and the resulting wetness of soils during warmer months, we assumed a somewhat higher rainfall recharge of 60% (0.65 m) at the spray site. The area of the spray site was assumed to be approximately 263,320 m2(=65 acres).
c) Over the course of a year, we assumed that, on average, 50% of the treatment plant effluent (446,000 gpd x 0.5 = 223,000 gpd) is spray irrigated (this was based on some old notes of mine, I do not have recent data).
d) Because spray irrigation occurs only during certain times of year, is not uniformly distributed over the spray area, and the soils at the site are often saturated (as evidenced by ponding of effluent that sometimes occurs at the site), we assumed 75% of the spray effluent water volume reaches groundwater (in contrast to a 50% recharge for rain).
With these assumptions in place, loading from the spray irrigation area was calculated as follows:
spray irrigation area N loading =
groundwater N [mg/l] x (volume spray recharge + volume rain recharge) /1,000,000 mg/kg
volume of spray recharge = 223,000 gpd x 3.785 l/g x 365 y/d x 0.75 recharge
= 2.31 x 108l/y
volume of rain recharge = area x recharge = 263,320 m2x 0.65 m
= 171,158 m3/y x 1000 l/m3= 1.71 x108l/y
spray irrigation area N loading/y = 12 mg/l x (4.0 x 108l/y) / 1,000,000 mg/kg
= 4,801 kg/y
As noted above, we assumed that less nitrogen is removed from the effluent during passage through the infiltration basins than by the spray irrigation area. After all, the whole purpose of the spray irrigation was to provide added nitrogen removal because infiltration basins are not effective in this process. Nitrogen concentrations under the spray irrigation area have ranged 6-12 ppm. CDM, the facility designer, expected nitrogen in groundwater under the spray irrigation area to be between 5 and 15 ppm(1), and as a result of this model, CDM recommended expanding the state Class III area to protect drinking wells. In CDM’s groundwater modeling of the site, under both the 5 and 15 ppm spray area groundwater scenarios, a plume exceeding 25 ppm was expected down gradient of the infiltration basins. Infiltration basin plume nitrogen concentrations were expected to be considerably higher than the spray area because the basins were expected to be less efficient at nitrogen removal, and because the effluent from the basins is less diluted by rainfall than in the spray irrigation area. Under the various modeling scenarios, Well 2 was at the periphery of the infiltration basin plume and instead reflected concentrations of the adjoining spray irrigation area (see attached figure B-10 from CDM’s 1987 report). Furthermore, after well installation, CDM noted a southerly component to the plume migration, a groundwater direction recently corroborated by the Cape Cod Commission in its new subwatershed delineation. The values of nitrogen in Well 2 have been consistently between 8 and 13 ppm DIN since 1991, but not as high as CDM’s modeling or as expected. These lower than expected values could largely reflect groundwater quality from the adjoining upgradient spray area. These results, together with the apparent peripheral location of Well 2 to the center of the infiltration basin plume has led us to believe that groundwater concentrations of nitrogen in Well 2 may not be adequate to estimate loadings from the infiltration basin.
In light of these questions surrounding Well #2, we have used an infiltration plume concentration of 22 ppm to estimate loading from the infiltration area. This value is intermediate between CDM’s worst case value of 32 ppm basin groundwater discharge and the 12 ppm peak observed under the spray irrigation area.
With these assumptions, infiltration basin loading, including consideration of rain recharge (in 5 acres of basins, assuming 90% recharge for both rain and effluent) would be:
infiltration basin N loading =
groundwater N [mg/l] x (volume basin discharge + volume rain recharge) /1,000,000 mg/kg
volume of basin discharge = 223,000 gpd x 3.785 l/g x 365 y/d x 0.90 recharge
= 2.77 x 108l/y
volume of rain recharge = area x recharge = 20,255 m2x 0.972 m (90 % rain recharge)
= 19,688 m3/y x 1000 l/m3= 1.97 x107l/y
infiltration basin N loading/y = 22 mg/l x (2.97 x 108l/y) / 1,000,000 mg/kg
= 6,529 kg/y
Clearly the estimate of the nitrogen concentration of infiltration basin discharge is one of the most critical assumptions in our nitrogen loading assessment. In our last phone call you mentioned you had funding to install 3 additional wells in the West Falmouth watershed. I would like to meet with you and Ed Eichner of the Cape Cod Commission to discuss potential sites and depths. My initial thoughts would be to site the wells as follows:
1) One well could be placed at the top of the water table immediately adjacent and down gradient (SW) of the infiltration basins. [Assuming, of course, that there is consensus that Wells 2 and 2a are inadequate for this purpose. The utility of Wells 2 and 2a will depend on the degree of mounding and lateral movement of recharge from the infiltration basins, which was predicted to be modest in the CDM groundwater model].
2) One well could be placed near Wells 11 and 11a, but at a shallower depth.
3) One well could be placed near Snug Harbor intermediate between Wells 13 and 14, and sited in a way to get minimal impact from septic system loadings and at a depth to be determined.
I hope this summary makes clear ours assumptions and the derivation of our estimates. As I noted in our progress report and cover letter, these were preliminary calculations and we hope to work with you to revise these assumptions to refine our loading estimates from the treatment plant.
I have also enclosed a diskette with a Lotus 1-2-3 spreadsheet file that includes all the calculations described above (see also attached printout). I have written the spreadsheet so you can plug in your own assumptions and constants so you can conduct a sensitivity analysis using this loading model. You will find that the loading model is most sensitive to changes in groundwater plume concentrations, followed by per cent of water directed to the spray area, and assumed recharge rates.
Please do not hesitate to call me if I can be of further assistance.
Joseph E. Costa
cc. Brian Currie, Planning Department
Rhett Lamb, BBAC representative
Ed Eichner, Cape Cod Commission
Camp Dresser and McKee. 1987. Summary of groundwater investigations in support of land disposal of treated wastewater from the Falmouth wastewater facility. Draft, January 1987.