Water Quality

2.2 Water Quality

The discussion below is drawn primarily from the CWMP Needs Assessment as it is based upon the most recent data available. At times cross-references to the Monomoy Study and the 2002 LCP are provided for historical perspective.

The CWMP estimated wastewater flows using the 2004 water data. Future flows took the 2004 base condition and combined that with the build-out to estimate future wastewater flows and wastewater nitrogen loadings. The CWMP nitrogen loadings represent a “worst case scenario” or “no-action.” This represents what might happen if the town did not take steps to change its handling of waste water.

Table 2-2-1, drawn from the CWMP, summarizes estimated wastewater flows and wastewater related nitrogen loadings.

Table 2-2-1 – Estimated Existing and Future Wastewater Flows and

Nitrogen Loadings 1,3Landuse	Existing Wastewater (gpd)	Future Wastewater (gpd)	Existing TN Load (kg/y)	Existing TN Load (lbs/yr)	Future TN Load (kg/y)	Future TN Load (lbs/y)
Multiuse 2	43,000	73,000	2,100	4,900	3,500	7,700
Residential	1,551,000	1,832,000	74,500	164,200	88,200	194,500
Commercial	240,000	379,000	11,000	24,300	17,700	39,000
Industrial	5,000	23,000	200	400	1,100	2,400
Institutional	51,000	98,000	2,500	5,500	4,700	10,400
Total	1,890,000	2,405,000	90,300	199,300	115,200	254,000
Notes: 1. Flows rounded to the thousands, nitrogen loadings rounded to the hundreds. 2. Multiuse properties include those with both residential and commercial use. 3. Future conditions based on build-out numbers and assume 35 mg/L (ppm) TN discharge from septic systems

The CWMP also provide us with an analysis of the impacts of waste water and nitrogen on the town’s coastal embayments. The Needs Assessment is only a preliminary analysis, which will be supplemented by additional analysis currently underway.

Nitrogen loadings to coastal embayments are a major concern on Cape Cod. Currently, sampling is being conducted in three Dennis embayments (Bass River, Swan Pond River and Chase Garden Creek). The current efforts involved in evaluating these embayments include:

• Sample and analyze estuary water for 3 summers

• Measure tidal flushing

• Study eelgrass and sediment to determine ecological health of the estuaries

• Develop a water quality model

• Identify the nitrogen limits

• Prepare the Nitrogen Limit Report

Table 2-2-2 provides a summary of the assumptions related to nitrogen loading in watersheds used in the CWMP analysis.

The CWMP used nitrogen loadings associated with wastewater, fertilizer, road runoff, roof runoff, natural areas, and surface water recharge were drawn from analyses used in other towns. The CWMP assigned flows and nitrogen loading on the watershed and embayments using the information in Table 2-2-2 and the wastewater flows (existing and future) listed above. Nitrogen loadings were developed for each of the coastal embayment watersheds as delineated by the CCC.

Table 2-2-2 – Nitrogen Loading Assumptions

Wastewater 2	Assumed Value 1
• Typical Title 5 Septic System	35 mg/L (ppm) total nitrogen
• Advanced Septic System	19 mg/L (ppm) total nitrogen
Fertilizers 3
• Typical Residential Lawn Area	5,000 sf
• Residential Application Rate	0.00108 lbs/sf
• Golf Course Application Rate	0.003 lbs/sf
• Parks/School Application Rate 4	0.002 lbs/sf
• Leaching Rate	20%
Impervious Surfaces
• Road Width 5	24 feet
• Road Runoff Concentration	1.5 mg/L (ppm) total nitrogen
• Roof Area 6	2,000 sf
• Roof Runoff Concentration	0.75 mg/L (ppm) total nitrogen
• Recharge Rate from precipitation	40 in/year
Water Bodies
• Concentration of precipitation	1.09 mg/L (ppm) total nitrogen
• Recharge Rate from precipitation	40 in/year
Natural Areas
• Recharge Rate from precipitation	26.5 in/year
• Concentration of precipitation	0.072 mg/L (ppm) total nitrogen

Table 2-2-3 – Existing Nitrogen Loading to Dennis’

Coastal Embayment Watersheds in kg/year (lb/y)

1Embayment	Fertilizer	Total Impervious 2	Waterbodies 5	Natural Areas	Wastewater	Total
Chase Garden Creek	800 (1,800)	1,000 (2,200)	0	300 (700)	12,800 (28,200)	14,900 (32,800)
Sesuit Harbor	700 (1,500)	900 (2,000)	0	200 (400)	9,600 (21,200)	11,500 (25,400)
Upper Bass River	3,700 (8,200)	1,200 (2600)	400 (900)	400 (900)	12,300 (27,100)	18,000 (39,700)
Lower Bass River	1,300 (2,900)	1,500 (3,300)	300 (700)	300 (700)	18,000 (39,700)	21,400 (47,200)
Kelleys Bay	400 (900)	400 (900)	0	100 (200)	3,800 (8,400)	4,700 (10,400)
Swan Pond River	1,300 (2,900)	1,400 (3,100)	100 (200)	300 (700)	17,000 (37,500)	20,100 (44,300)
Quivett Creek 3	100 (200)	100 (200)	0	0	800 (1,800)	1,000 (2,200)
Herring River 4	100 (200)	100 (200)	0	0	1,100 (2,400)	1,300 (2,900)
Notes: 1. Loadings rounded to the hundreds. Values of zero represent less than 50 kg/year (110 lbs/year). 2. Total impervious equals the sum of the road runoff load and the roof runoff load. 3. The majority of the Quivett Creek embayment is located within the Town of Brewster. 4. The majority of the Herring River embayment is located within the Town of Harwich. 5. Waterbodies represent the nitrogen loading from atmospheric deposition directly on a waterbody.

 Table 2-2-4 – Future Nitrogen Loading to Dennis’

Coastal Embayment Watersheds in kg/year

Embayment	Existing Non-wastewater Load 1	Additional Roof Load 2	Additional Fertilizer Load 2	Future Wastewater Load 1	Total 1
Chase Garden Creek	2,200 (4,900)	30 (70)	90 (200)	14,500 (32,000)	16,800 (37,000)
Sesuit Harbor	1,800 (4,000)	10 (20)	50 (110)	11,200 (24,700)	13,100 (28,900)
Upper Bass River	5,600 (12,300)	10 (20)	50 (110)	13,400 (29,500)	19,100 (42,100)
Lower Bass River	3,400 (7,500)	30 (70)	110 (240)	25,400 (56,000)	28,900 (63,700)
Kelleys Bay	800 (1,800)	0	20 (40)	4,000 (8,800)	4,800 (10,600)
Swan Pond River	3,100 (6,800)	20 (40)	70 (150)	25,200 (55,600)	28,400 (62,600)
Quivett Creek 3	300 (700)	0	0	1,000 (2,200)	1,300 (2,900)
Herring River 4	100 (200)	0	0	3,400 (7,500)	3,500 (7,700)

Regardless of what watershed reviewed, the findings are pretty stark. Existing and future nitrogen loads are 80 to 90 percent due to wastewater generation. The additional load generated from new roofs and new lawns created by subdivisions are minimal, even if the assumed average areas of 2,000 sf and 5,000 sf respectively are doubled or tripled.

Dennis, as well as the entire Cape, depends almost entirely on groundwater for its drinking water. The Monomoy groundwater lens is one of six ground water lenses on Cape Cod, and Dennis main source of water, which together are referred to as Cape Cod’s Sole Source Aquifer. Since groundwater is the only source of drinking water for Dennis, and Cape Cod, thorough protection measures are required to ensure the preservation of this natural resource for present and future residents and visitors.

One of the primary contaminants of concern to groundwater is nitrogen from residential septic systems. High levels of nitrogen in groundwater threaten to destroy this unique resource as a source for Dennis’ drinking water. Nitrate-Nitrogen (NO₃-N) in groundwater is a health concern. There are direct health risks associated with ingesting excessive NO₃-N levels, including blue-baby syndrome. Also, high levels of NO₃-N levels indicate the water supply is intercepting septic system leachate and may be contaminated with other harmful chemicals contained in wastewater.

The current quality of groundwater in the Monomoy Lens, which includes Dennis, is relatively pristine. Nitrate occurs naturally in the environment at low concentrations, around 0.05 milligrams per liter (mg/l), from the decay of vegetation and the infiltration of nitrogen-laden precipitation. Levels over 1.0 parts-per-million (ppm) demonstrate the impacts of land development and other human activity on water purity.

The impact of nitrogen within the Zone IIs of the Town’s public drinking water supplies is also of importance. The Federal and State limits for nitrates in drinking water are currently 10 mg/L (ppm). The CCC has also established a goal of maintaining nitrate concentrations in groundwater at less than 5 mg/L (ppm) because of the sole source nature of the aquifer.

The CWMP calculates nitrogen loading for the eleven Zone IIs in Dennis in a manner similar to that of the watersheds. Due to overlap in the boundaries, the CWMP analysis considers each well in isolation, providing an area specific analysis as if there were no other wells drawing from the recharge zone.

Table 2-2-5 – Existing Nitrogen Loading to Dennis’ Zone IIs in kg/year (lbs/year) ¹

Zone II	Well Number	Fertilizer	Total Impervious2	Waterbodies	Natural Areas	Wastewater	Total
1	Yarmouth3	0	0	0	0	100 (200)	100 (200)
2	19, 20	2,800 (6,200)	700 (1,500)	200 (400)	200 (400)	7,400 (16,300)	11,300 (24,900)
3	14,15	200 (400)	200 (400)	100 (200)	100 (200)	2,200 (4,900)	2,800 (6,200)
4	1, 2, 3, 10, 12, 16	100 (200)	300 (700)	100 (200)	100 (200)	1,900 (4,200)	2,500 (5,500)
5	9	1,300 (2,900)	300 (700)	100 (200)	100 (200)	2,800 (6,200)	4,600 (10,100)
6	7, 8	100 (200)	100 (200)	0	100 (200)	1,100 (2,400)	1,400 (3,100)
7	5, MS	100 (200)	300 (700)	0	100 (200)	2,200 (4,900)	2,700 (6,000)
8	4, 6, 11	100 (200)	200 (400)	100 (200)	100 (200)	2,100 (4,600)	2,600 (5,700)
9	18	1,200 (2,600)	200 (400)	0	100 (200)	2,600 (5,700)	4,100 (9,000)
10	21	200 (400)	300 (700)	0	100 (200)	2,600 (5,700)	3,200 (7,100)
11	22	1,300 (2,900)	300 (700)	100 (200)	100 (200)	3,200 (7,100)	5,000 (11,000)
Notes: 1. Loadings rounded to the hundreds. Values of zero represent less than 50 kg/year (110 lbs/year). 2. Total impervious equals the sum of the road runoff load and the roof runoff load. 3. The majority of this Zone II and well are located within the Town of Yarmouth.

Table 2-2-6 – Future Nitrogen Loading to Dennis’ Zone IIs in kg/year (lbs/year)

Zone II	Well Number	Existing Non-wastewater Load1	Additional Roof Load2	Additional Fertilizer Load2	Future Wastewater Load1	Total1
1	Yarmouth3	0	0	10 (20)	200 (400)	200 (400)
2	19, 20	4,000 (8,800)	10 (20)	30 (70)	8,500 (18,700)	12,500 (27,600)
3	14,15	600 (1,300)	0	10 (20)	2,500 (5,500)	3,100 (6,800)
4	1, 2, 3, 10, 12, 16	600 (1,300)	10 (20)	20 (40)	2,200 (4,900)	2,800 (6,200)
5	9	1,800 (4,000)	10 (20)	10 (20)	3,100 (6,800)	4,900 (10,800)
6	7, 8	300 (700)	0	10 (20)	1,300 (2,900)	1,600 (3,500)
7	5, MS	500 (1,100)	10 (20)	30 (70)	2,800 (6,200)	3,300 (7,300)
8	4, 6, 11	500 (1,100)	10 (20)	20 (40)	2,400 (5,300)	2,900 (6,400)
9	18	1,500 (3,300)	0	10 (20)	3,300 (7,300)	4,800 (10,600)
10	21	600 (1,300)	10 (20)	30 (70)	3,100 (6,800)	3,700 (8,200)
11	22	1,800 (4,000)	10 (20)	20 (40)	3,900 (8,600)	5,700 (12,600)
Notes: 1. Loadings rounded to the hundreds Values of zero represent less than 10 kg/year (20 lbs/year). 2. Future roof, lawn and natural area loadings represent the net change and are rounded to the tens. 3. This well and the majority of its Zone II are located within the Town of Yarmouth.

Nitrate concentrations greater than 2 mg/L (ppm) were found in seven wells (Nos. 2, 3, 5, 14, 16, 21, and the Main Station) over a three-year sampling period (2001 -2003). None of these wells have exceeded the 5 mg/L (ppm) or 10 mg/L (ppm) levels as identified by the CCC and MADEP respectively. Although no correlation was seen between the nitrogen loadings (or nitrogen loading per area) and the nitrate concentrations, the land uses adjacent to these areas were examined as possible sources. Zone II overlaps, depth of wells, pumping rates, groundwater time of travel, and local land uses all impact the nitrate concentrations found in each well. USGS modeling as described in the next section provided some advanced modeling so these loading impacts could be evaluated further.

The following table summarizes the USGS modeling results for the existing contributing areas for the seven wells.

Table 2-2-7 – Summary of USGS Modeling Results for Seven Wells

Well Number	Maximum Nitrate Concentration (mg/L)2	USGS Estimated Septic System Flow (gpd)	Estimated Nitrogen Loading kg/year (lbs/year)
2	2.65	4,000	200 (400)
3	2.64	0 1	0
5	3.46	10,800	500 (1,100)
14	2.50	6,700	300 (700)
16	3.11	4,100	200 (400)
21	2.00	17,600	800 (1,800)
Main Station	3.90	5,400	300 (700)
Notes: 1. Proximity to Well No. 2 may have impacted this value. 2. mg/L = ppm

No true correlation has been established between the estimated septic nitrogen load and the nitrate concentrations. However, of all the wells showing elevated nitrate concentrations, their relative distance to the nearest residential development was the smallest. The Main Station wells and Well No. 5 are also the closest to major roadways (Route 6 and Route 134, respectively) and therefore may also be impacted by stormwater runoff from these major roadways.

Revised November 18, 2011