AWWA Water Resources Spring Seminar 2011

April 12, 2011

Niffy Saji

Fairfax Water

Occoquan Reservoir: Source Water

Protection and Planning

The Occoquan Reservoir

� The Occoquan Reservoir is bordered by Fairfax County on the north, and Prince William County on the south.

� Man made impoundment

� Primary source of drinking water to more than 1.7 million residents who live in Northern Virginia.

� It is a run-of-the-river reservoir

Occoquan Reservoir Facts

� Volume 8.3 billion gallons

� Surface Area 1,538 acres

� Length 14 mi.

� Mean Depth 16 ft.

� Maximum Depth 65 ft.

� Maximum Width 900 ft.

� Dam Height 122 ft. above mean sea level

� Mean Hydraulic Residence Time – 19 days

Arial of High Dam and Reservoir

Occoquan Watershed Facts

� Area: 590 Square Miles

� Streams: 1,300 Miles

� Major impoundments:� Lake Manassas� Lake Jackson� Occoquan Reservoir

� Residents: 400,000 (estimate)

� Land Use (estimate):� 14% Low Density Residential� 8% Medium and High Density Residential� 4% Commercial and Industrial� 1% Institutional� 13% Agricultural� 9% Pasture� 51% Forest and Idle

Source Protection Initiatives

� Occoquan Policy

� Occoquan Non-Point Source Program

� Fairfax County Water Supply Overlay District

� Fairfax County Down-zoning

� Occoquan Shoreline Easement Policy

� Water Supply Outreach Grant Program

The Occoquan Policy

� Result of late 1960s Water Quality Problems

� Established Occoquan Watershed Monitoring Program

� Formed UOSA (Upper Occoquan Service Authority) to Consolidate Older WWTPs

� Established Indirect Potable Reuse as Sustainable Water Supply for Northern Virginia

Occoquan Watershed Monitoring

Program

� Occoquan Watershed

Monitoring Subcommittee

� Occoquan Watershed

Monitoring Lab (OWML)

� Sampling and Analysis

� Stream Gages and Reservoir

Stations

� Objective: to Evaluate and

Monitor Long-Term Water

Quality Trends in the

Watershed

Occoquan NPS Programs

� Technical Investigations and Support

� Northern Virginia BMP Handbook

� Nonstructural Urban BMP Handbook

� Land Use Inventory and Updates

� Occoquan NPS Model

� Historic Fairfax County Down-zoning to Protect Occoquan Reservoir

� Fairfax County Water Supply Overlay District

Shoreline Easement Policy

� Fairfax Water has a shoreline easement around the Reservoir

� The policy defines activities permissible within the easement

� Land disturbance activities require prior permission from Fairfax Water

Shoreline Easement Policy

What’s NOT Allowed

� (New) Structures other than Piers or Floats

� Clearing of Trees and Vegetation

� Closed Storage Sheds

� Fuel and Chemical Storage

� Application of Pesticides, Herbicides, and Fertilizers

� Trash, Debris, and Vegetative Waste

� Pipes Withdrawing or Discharging Water

� Septic Tank Installation

Shoreline Stabilization

� Shoreline stabilization and erosion control projects are permitted with prior permission of Fairfax Water

� Approval of local, state and federal regulatory agencies are required

� Vegetative stabilization practices are required unless technical considerations justify more hardened practices.

Watershed and Water Supply

Outreach Grants

Fairfax Water offers watershed and water-supply education grants to support community efforts to protect our source water and supply. Eligible projects include

� Education Efforts� Source-Water Protection Projects� Water Quality Monitoring Projects� Occoquan Reservoir Shoreline Stabilizations.

To improve Reservoir water quality near the water supply intake, FW is replacing a

Destratification aeration system with a Hypolimnetic Oxygenation System (HOS)

Existing System:

Destratification�Goal: break stratification & mix entire water column

�Compressed Air (150 SCFM)

�Diffuser: 32 aeration lines (7,500 ft)

�Aeration lines lay on reservoir bottom

New System:

Oxygenation (HOS)�Goal: aerate deep waters only

�Pure oxygen (50 SCFM)

�Diffuser: single line (2,500 ft: in former

riverbed )

�Diffuser is suspended off reservoir bottom

Reservoir Management

Despite the existing Destratification system, 70% - 80% of water column at Occoquan

Dam has low DO levels during thermally stratified months (2002 – 2007 data)

� Plots show: DO < 1 mg/L (Red), DO < 2 mg/L (Green) and DO < 5 mg/L (Blue)

60

70

80

90

100

110

120

130

1-Jan-02 1-Jan-03 1-Jan-04 1-Jan-05 1-Jan-06 1-Jan-07 1-Jan-08

Date

Ele

vat

ion

(ft

) _

DO < 1 mg/l DO < 2 mg/l DO < 5 mg/l WSE Aerators ON Estimated Stratification

Top of Dam

(122 ft msl)

Reservoir

Bottom

(~60 ft msl)

Reservoir Management

A Hypolimnetic Oxygenation System (HOS) aerates the oxygen-deficient

zone (Red area) without trying to mix the Reservoir water column……..“works

with Mother Nature”

Bottom Intake (80 ft)

Middle Intake (95 ft)

Top Intake (110 ft)

+

Top of Dam (122 ft)

+

+

112 ft

100 Ft

Epilimnion

Thermocline

Hypolimnion

Key

Hypolimnetic Aeration System

The new Occoquan Reservoir Oxygenation system offers several benefits

� Will achieve improved Reservoir water quality near the Dam and the water supply intake� Replaces undersized Destratification system (1970s aeration technology - compressed air)� Can keep bottom DO above 2 mg/L (controls sediment releases of Manganese)� Can reduce bottom sediment releases of Phosphorus (controls algae blooms)

� Cost-effective Reservoir management alternative to achieve FW’s water quality goals � Can save up to $700,000/year in Potassium Permanganate treatment cost

� Oxygenation system Payback Period is 2–4 yrs

� A more consistent and robust pretreatment step for the Reservoir water supply

� HOS system is expected to be online in 2012

Summary: Oxygenation System

Questions/Comments