1995 letter to Falmouth, MA Wastewater Facilities Manageroceanscience.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
Explanation of loading estimate:
Because there are many information gaps regarding loading from the plant, we made the following
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
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
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.