What are the nitrogen levels?
  1. Current levels in our watersheds vs. allowable levels

    The table below provides the current attenuated nitrogen load in each of the watersheds within the Town of Dennis compared to the threshold nitrogen load. The threshold load is the amount of a substance that can be assimilated into a natural environment to maintain the water body's designated uses:

    WatershedCurrent Attenuated Total Loading (kg/day)
    Threshold Total Loading (kg/day)
    Bass River
    Swan pond
    Herring River
    Chase Garden Creek
    Sesuit Harbor
    Results pending release of final MEP report
  2. Healthy drinking water

    2020 Annual Range of nitrate as N found in drinking water supply wells: Non detect up to 2.1 mg/L vs. a Maximum Contaminant Level (MCL): 10 mg/L.
  3. Title 5 septic system output

    Effluent from a Title 5 septic system contributes approximately 6.2 kg of nitrogen/yr/parcel, leaving a nitrogen concentration in the recharged effluent at a concentration of about 35 mg/L.
  4. I/A system output

    Effluent from an advanced innovative and alternative (I/A) wastewater treatment system contributes approximately 3.1 kg of nitrogen/yr/parcel, leaving a nitrogen concentration in the recharged effluent at a concentration of 13 to 19 mg/L.
  5. PRB attenuation

    The Town of Dennis has included permeable reactive barriers (PRBs) in their wastewater management plan. These barriers would provide increased nitrogen removal from the watersheds. For the sites that include PRBs, the intention would be to install them around the perimeter of the effluent recharge basins as a final polishing step or downgradient of Town owned landfills and other sites that are suspected of contributing high concentrations of nitrogen to the surrounding watersheds. Site specific investigations will need to be performed in order to determine the PRB's effectiveness in removing nitrogen at each site. The treatment technologies chosen for application at the WWTP are capable of achieving an effluent nitrogen concentration of 3 mg/L, which is required by the TMDL set in the MEP reports for the Bass River and Swan Pond River watersheds. As stated in the CCC 208 plan, treatment of wastewater through a PRB can result in a 75% to 95% reduction in the concentration of nitrogen in the effluent. Assuming that the application of a PRB around effluent recharge ponds will result in an effluent nitrogen concentration of 1 mg/L is a conservative approach, as this would indicate that the PRB was capable of reducing nitrogen by 67%.
  6. What is Permeable Reactive Barrier (PRB)?

    A PRB is a subsurface permeable barrier, comprised of either inorganic or organic material, chosen to target the reduction of specific contaminates. This material provides a carbon source for denitrification to occur
  7. Natural Attenuation

    Values of natural nitrogen attenuation vary greatly depending on the media the wastewater travels through, including ponds, streams and marshes. Natural attenuation can account for anywhere between 30 to greater than 50% nitrogen attenuation depending on the path the groundwater travels prior to entering the estuaries.
  8. Allowable levels of nitrogen to restore our estuaries are defined above. Is there value in seeking interim values that could be more affordable but only achieve some fraction of the total reduction needed?

    The rate at which the estuaries are deteriorating will already take many years to slow down and ultimately reverse the impacts, even if immediate action is taken; therefore is it recommended that the full threshold values be targeted. However, the town wide plan is likely to be implemented in phases within each watershed and the water quality monitored between phases to determine progress in meeting the targeted values.

Show All Answers

1. Why all the talk about wastewater?
2. I thought Title 5 septic systems solved the issue of increased nutrient loading from wastewater on groundwater?
3. What are the nitrogen levels?
4. What solutions to wastewater contamination are being considered for Dennis?
5. Do septic systems impair our fresh water bodies?
6. What is the Cape Cod Commission 208 plan?
7. What are funding impacts?
8. How many phases will there be in the 40 year sewer project?