D. 3D_J001B DP3FINAL WELLHD PROTECT PLAN 04JAN19.pdf

NAVFAC PACIFIC/MARIANAS

WELLHEAD PROTECTION PLAN

FINAL SUBM ITTAL

10/23/ 2018

TABLE OF

CONTENTS 1 PROJECT LOCATION………………………………… 1

2 PROJECT DESCRIPTION…………………………………2

3 PURPOSE

4 STORM WATER DRAINANGE……………………

4.1 STORMWATER REQUIREMENTS ................. …….5

4.2 DESIGN APPROACH

5 CONCLUSION

6 REFERENCES …………………………………………9

7 APPENDIX

APPENDICES

A EXHIBIT A: WELL HEADS IMPACTED BY THE NEW SITE LAYOUT

B EXHIBIT B: CONFLICTING HIGH RISK ACITVITIES WITH WELL HEAD

PROTECTION AREAS

C EXHIBIT C: ONSITE INFILTRATION BASIN/SWALE DETAILS

1 PROJECT LOCATION

The site is located in Finegayan in Northern Guam, adjacent to the junction of Route 3 and Route 3A.

2 PROJECT DESCRIPTION

The J-001B USMC Main Cantonment project is a new operations base for the Marine Corps in Guam.

The project is intended to provide grading, drainage, utilities and roads to support future development in the central area of the Marine Corps Cantonment site. The site is located in Northern Guam within the limestone bedrock dominated region. The project includes storm water treatment and conveyance, water distribution, sewer services, electrical services, comm unication lines and roadways. To facilitate these utilities a water tank and treatment facility, lift stations, and an electrical substation are included in the project. The project also includes parking spaces along one portion of the new roadway system. The project will be served by six infiltration basins, three within the wellhead protection zone (Basins 1,2 and 4) and three outside of the wellhead protection zone (Basins 3,5 and 6). All six basins will be lined.

In accordance with 22GAR-7 §7130 – Wellhead Protection for Public Water Supply Wells projects that are partially or entirely located within 1,000 ft radius of any public utility potable water supply well, must obtain Guam Environmental Protection Agency (GEPA) approval to construct such facilities. This area within the 1000 ft radius is defined as the wellhead protection zone. It is the intent of this report to present the strategy that will be implemented within the wellhead protection zone to protect and preserve groundwater quality.

3 PURPOSE

The purpose of this report is to provide guidelines to

1- Preserve the potable wells within the project boundaries and outside the limits of the project area

2- Restore to an equivalent or improved condition of potable water wells upon completion of work

3- Confine construction activities to within the limits of the work indicated or specified 4- Avoid incompatible development and incompatible land use within wellhead protection zones during construction and operations.

5- Provide a summary of the stormwater management strategy within the wellhead protection area to address stormwater disposal concerns.

There are four existing Navy water wells within the project site, an offsite Navy and offsite GWA water well have wellhead protection zones that overlap the construction site, those wells are indicated in Exhibit A.

Around each well is a 305 m (1,000ft) radius wellhead protection zone in which stormwater basins and high risk pollution hot spots should be minimized and storm water treated for pollution in order to protect the water quality of the aquifer serving the wells. Four existing wells will be adjusted due to site grade changes; existing wells shall remain operational and only one well can be taken out off-line at a time for extended outages.

Exhibit B identifies high risk activities or land use within the wellhead protection area (WHPA) that meets any of the following categories:

A. Vehicle fueling stations B. Vehicle service and maintenance facilities C. Vehicular and equipment cleaning facilities D. Outdoor loading or unloading facilities (storage, warehouse, comm ercial) E. Facilities generating or storing liquid waste, solid waste, or hazardous waste F. Dining/ food preparation facilities G. Engineering training pit (outdoor area to train operators using earth moving or other large equipment) H. Storm water disposal facilities (unlined storm water basins) I. Parking Lots of more than 12 stalls

4 STORM WATER DRAINAGE

The proposed grading associated with the project will divide the site into six drainage areas each served by a regional basin. Three of the five basins (Basins 1,2 & 4) are located partially or wholly within the wellhead protection areas. In order to protect the wellheads, storm water will be treated prior to reaching groundwater zones. An integrated treatment control strategy that includes bioswales/bioretention trenches and biofiltration within infiltration basins will be incorporated into the project design to ensure the storm water pollutants are treated adequately prior to recharge to the aquifer.

Initial pollutant sources include parking lots and roadways, which will be expanded to include other land uses as the project area develops. Anticipated pollutants include pathogens, metals, nutrients, organic compounds, sediments, trash, oil & grease.

Bioretention also known as biofiltration will be the primary strategy used for stormwater treatment and has been demonstrated to be a highly effective stormwater treatment strategy that relies on physical, chemical and biological processes to treat stormwater runoff. This treatment strategy uses the natural removal processes of soil during infiltration of stormwater. For pollutant removal, bioretention is the best practice amoung BMPs (William Hunt, 2012 Stormwater Report). Common particles removed from stormwater via bioretention include particulate organic matter, phosphorus, and suspended solids. Biological processes that occur result in pollutant uptake by microorganisms in the soil. Microbial activity within the soil also contributes to the removal of nitrogen and organic matter. Nitrogen is removed by nitrifying and denitrifying bacteria, while aerobic bacteria contribute to the decomposition of the organic matter.

The removal effectiveness of bioretention has been studied during field and laboratory studies conducted by the University of Maryland (Davis et al, 2001). Removal for typical stormwater pollutants are shown in Table 1.

Table 1. Bioretention Removal Effectiveness (Davis et al. 2001)

Bioretention has also been studied and shown to function effectively overtime (Lucke and Nichols, 2015).

While local Guam manuals allow for multiple options for stormwater treatment, bioretention will be used as the primary treatment strategy due to the effectiveness to remove pollutants from stormwater and preserve groundwater quality.

4.1 STORMWATER REQUIREMENTS

In accordance with UFC 3-210-10 N the storm water treatment strategy will be designed to capture and treat 95% of the average annual runoff 55.9 mm (2.2 inches). Per the CNMI and Guam Stormwater Management Manual, the post construction storm water management strategy shall meet Recharge Criteria, Water Quality Criteria, Channel Protection Criteria and Flood Control Criteria.

The primary process for treatment will be through infiltration/biofiltration through proposed engineered soil media. The 95% percentile runoff will be completely retained and filtered. Biofiltration swales that will be constructed with check dams will provide retention of the 95% percentile runoff and allow runoff to percolate through an engineered soil media that will comprise of organics, clay, silt and sand. This strategy is high removal rates for stormwater pollutants.

4.2 DESIGN APPROACH

In accordance with UFC 3-210-10N and the CNMI and Guam Stormwater Management Manual, The 95% storm will be completely captured and treated within the roadside V and trapezoid swales, the 1 year storm will be fully captured with swales and infiltration basins, and the difference between the pre and post construction runoff for the 25 year storm will be captured within the infiltration basins. The proposed grading will divide the site into six drainage areas each served by an infiltration basin (See Figure 1).

Figure 1. Drainage Basins

All basins will be lined with bioretention soil media to promote the removal of stormwater contaminants.

Stormwater will percolate through the engineered soil media where pollutants will be removed via adsorption, absorption and ion exchange. The basin bottoms will consist of 760 mm (30 inches) of soil media for stormwater treatment, underlain by 50 mm (2 inches) of class 2 permeable and 100 mm (4 inches) of gravel to allow for equal distribution of infiltration into underlying bedrock.

Figure 2. Lined Basin Typical Section

Roadside swales will consist of v shape or trapezoidal shape and will also be lined with a surface layer of either grass/mulch or limestone coral rock depending on the slope and velocity. All swales sections will include a minimum of 760 mm (30 inches) of soil media to promote the removal of stormwater contaminants (see Figure 3&4). Check dams will be placed intermittently to promote retention and subsequent filtration through the engineered soil media. Two V shaped swales outside of the wellhead protection zone do not have treatment media (north side of Road 5 between Roads 2 and 6). The perimeter road swales are also not required to have treatment media.

Figure 3. Lined V Swale Typical Sections

This design approach will address runoff from the roadway, future lot developments and the proposed infrastructure buildings for wells, pump stations and water treatment. Bioretention within the roadside swales is sized to treat the 95% percentile runoff. Therefore it can be concluded that only 5% of average annual rain events will exceed the swale treatment capacity and spill to the basins. The 1 year 24 hour storm is completely retained within a combination of the swales and infiltration basins. The basins have also been sized to retain the difference between the pre and post construction runoff for the 25 year storm.

Figure 4. Lined Trapezoid Swale Typical Sections

CONCLUSION

The proposed integrated strategy outlined in this discussion, which includes bioretention/biofiltration for primary treatment of storm water within the wellhead zone and avoids incidental infiltration directly into bedrock will provide a highly effective treatment strategy for storm water pollutants while protecting groundwater quality. These treatment strategies comply with the CNMI/Guam Stormwater Management Manual and the UFC 3-210-10 N. Stormwater that has been treated will then infiltrate into underlying bedrock and allowed to percolate down to the groundwater zone. All Infiltration basins will be lined. Bioretention/biofiltration is a highly effective strategy for removing storm water pollutants and will ensure protection of groundwater quality.

This narrative has clearly presented the proposed conceptual treatment strategy within the wellhead zone and that stormwater will be treated with a highly effective strategy to preserve groundwater quality. Calculations for stormwater facilities is provided in the drainage report. An inspection and maintenance plan will also be developed to ensure the bioretention swales and basins are maintained to preserve functionality over the design life.

REFERENCES

2006 CNMI and Guam Stormwater Management Manual, October 2006

UFC 3�201�01 Civil Engineering, 01 June 2013

UFC 3�210�10N Low Impact Development, 6 April 2010

Best Management Practices for Wellhead Protection Northern Guam Lens Aquifer, December 2010

Conceptual Grading, Drainage, and Low Impact Development Guam and CNMI Military Relocation, August 2013

Soil Survey of Territory of Guam, May 1988

Site Characterization Study NCTS Finegayan, Guam, Mariana Islands, 31 July 2008

Hunt, William (2012, June) Why is Bioretention So Popular. The Stormwater Report

Davis, A.P., Shokouhian, M., Sharma, H. and Minami, C., “Laboratory Study of Biological Retention (Bioretention) for Urban Stormwater Management, “ Water Environ. Res., 73(1), 5-14 (2001).

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APPENDIX

A EXHIBIT A

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B EXHIBIT B

C EXHIBIT C

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