The Impact of Intensive Stormwater Infiltration on Groundwater Levels in PhiladelphiaPhiladelphia Water Department

AWRA Annual ConferencePhiladelphia November 1 – 4, 2010

Mark Maimone, Ph.D, P.E., D.WRE

Dan O’Rourke, PG

Presentation Overview

Infiltration through green infrastructure in urban settings is a national trend

Little known about impacts to the groundwater system

Two models developed for the Philadelphia Water Department to study impacts of their Green Infrastructure Program

Philadelphia’s Unique Plan for Combined Sewer Overflow Control

$1.6 billion spent over 20 to 25 years

$1.0 billion to “green” one-third of combined-sewer area by managing stormwater at the source

Green Infrastructure

“Greened Acres”

In order to achieve CSO capture objective, Philadelphia is planning to convert 9,500 impervious acres to Greened Acres

1 “Greened Acre” manages the 1st inch of runoff from 1

impervious acre prevents 250,000 gallons of overflow / year

Need for Groundwater Modeling

Philadelphia’s Program of stormwater infiltration in highly urban environment highly ambitious

Green infrastructure approaches are becoming very popular across the US for stormwater and CSO programs.

Impacts of concentrated infiltration in urban setting need to be evaluated

Critical Concerns

Extremely important to study impacts to groundwater prior to proceeding.

Will conversion of 34% of city’s impervious cover to a green design that captures 1st

inch of rainfall cause a long term rise in groundwater?

Will this higher water table cause basement flooding?

Groundwater Modeling at 2 Scales

Modeling on a local scale evaluated water table mounding on a city block basis Subject of the presentation following

this one

Larger scale model needed to evaluate city-wide water table increases associated with a 34% Green Program in Philadelphia

Philadelphia Water Table Map

Utilize 1980 Water Table for Model Extent & Boundary Conditions

Set Model Boundaries at Major Surface Water Features and Groundwater Divides

Lateral Edges

Delaware River

Schuylkill River

North near Route 1 (local groundwater divide)

Finite Element Model Grid

7,253 nodes

14,290 elements

Node spacing 250 feet to 1,000 feet

12 model levels

Model Grid

Model Stratigraphy and Hydrogeologic Framework

Model domain includes both the Coastal Plain & Piedmont physiographic provinces, divided by the Fall Line

Two very different hydrogeologic environments

from USGS WSP-2346, 1991

Data Sources:

USGS monitoring wells

PWD soil borings

Borings identified in WSP-2346 (USGS)

PWD Soil Borings

Used for depth to bedrock only

Stratigraphy from cross-sections in USGS reports

Data Points

Cross-Section through Coastal Plain

Cross-Section Location

Cross-Section through Piedmont

Stratigraphic Unit Boring Log Legend

Cross-Section Location

Recharge & Groundwater Pumping Assumptions

For baseline, steady-state groundwater model, recharge is uniformly assigned to the model at 17.6 inches/year From SWMM models

The model currently does not include any groundwater withdrawals for industrial use or any dewatering that is occurring for subway tunnels or other facilities (Citizens Bank Park, etc)

Incorporation of Sewers

From USGS WSP-2346

Construction of sewers in Philadelphia significantly changed the natural hydrologic cycle. Interchange of water from sewers with the ground-water system now represents a major flow path in the hydrologic cycle.

History (from USGS WSP 2346)

Sewer construction began ~ 1855 (brick) Sewers constructed before 1875 – no mortar

between brick to allow for groundwater infiltration to lower the water table and alleviate flooding Brick sewers that were built with mortar had

a lime based mortar which dissolved easily Brick sewers built until ~ 1940 Vitrified clay pipes Concrete pipes

From Stream to Sewer

Sewers often located in areas of previous streams, but have been filled in with coarse material

Sewer Impact on Water Table

Influence on water table evident from 1980 water table map

Baseline Simulation

Use current infiltration estimate for average annual rainfall year

Adjust boundary conditions and aquifer properties of steady-state model to match existing data

General flow consistent with published maps

Summary of Baseline Regional Model

Overall, the model is in general agreement with groundwater flow direction

Model suitable for exploratory simulations of water table mounding for Philadelphia’s Green Program

Simulated infiltration into sewers = 16 MGD under baseline (steady-state) conditions

Enhanced Recharge

Use baseline regional groundwater model to evaluate water table mounding resulting from implementation of a 34% Green Program

Areas of recharge determined from Philadelphia Infiltration Feasibility Map (PWD, 2009) Eliminate areas that have an infiltration score of >

0.50 Eliminate areas within 2,500 feet of model

boundaries (water table too shallow for infiltration)

Infiltration Suitability Scoring

Fill Location

No Fill 0

Fill 0.5

Depth to Ground Water (ft)

> 10' 0

5' < and < '10 0.2

< 5' 0.25

Soil Type

SoilA 0

SoilB 0.1

SoilC 0.2

Urban Soil 0.2

Soil B/D 0.2

SoilD 0.25

Water 1

Score Status 0 Infiltrate

>0 and ≤0.5 Geotech Investigation or Design Limitations

>0.5 and <1 Geotech and Design Limitations

≥1 Infiltration Not Recommended

Recharge Avoides “not recommended’ ares

34% Green Program = 5,291 ac

Enhanced Recharge Elements5,291 ac

Water Balance (2005 precipitation)

Areas with Green Infrastructure: 45.6 inches per year precipitation 8.1 inches to sewer from > 1 inch of daily

rainfall 2.1 inches to evaporation (est) 35.4 inches per year recharges groundwater

Areas without Green Infrastructure Recharge 17.6 inches

Utilized Steady-State Approach

Represents a “ceiling” to the regional water table mounding

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• Solid line: simulated with 15 min recharge input into transient model

•Dotted line: simulated using average daily recharge value

Green Infrastructure: Recharge or Slow Release yet to be determined

Based on 2005 precipitation data: 35.4 inches per year in greened area enters

Green Infrastructure

Balance between recharge and slow release will be determined by soil conditions

Conservative estimate used: 70% recharge, 30% slow release

High recharge estimate: 90% recharge, 10% slow release

Recharging 24.78 inches per year over 5,227 acres results in a maximum water table mounding of 5.7 feet

Limited water table mounding in Coastal Plain

Total of 17.25 MGD into sewers (or additional 1.63 from baseline)- 58% of enhanced infiltration

0.7” to groundwater0.3” to slow-release & evap

Recharging 31.9 inches per year over 5,227 acres results in a maximum water table mounding of 9.6 feet

Limited water table mounding in Coastal Plain

Total of 18.81 MGD into sewers (or additional 3.18 from baseline)- 57% of enhanced infiltration

0.9” to groundwater0.1” to slow-release & evap

Time to Steady-State

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Water Table Mounding in Coastal PlainFull Steady state after 16 years of recharge

Most mounding within first 5 years

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Water Table Mounding in Piedmont

Most mounding within first 10 years

Time to Steady-State Full Steady state after 22 years of recharge

SE Model Summary

Long term increases in the groundwater elevation can be managed by avoiding infiltration in areas of shallow groundwater

A significant percentage of the stormwater is likely to re-enter the sewers, but at a steadier, more controlled rate. For areas facing requirements to reduce CSOs, this is a beneficial effect.

SE Model Summary

In modeling urban groundwater systems, the interaction of infiltration, soil properties, and the design of green stormwater infrastructure interact in complex ways

Transient mounding effects near infiltration facilities are impossible to predict without using numerical models with transient capabilities. (discussed in next presentation)

SE Model Summary

The water table is usually lowered by impervious cover in cities as recharge is reduced. Green stormwater infrastructure can reverse this, and create enhanced recharge over natural recharge rate.

City-wide effects of enhanced recharge do occur over time, as the groundwater system seeks a new equilibrium.

Conclusions

An ambitious program such as Philadelphia’s can result in water table rises of up to 4-8 feet in some areas

Larger groundwater increases will occur in the Piedmont, where depth to groundwater is the greatest

Smaller increases of less than 2 feet will occur in Coastal Plain

In almost all of the city, long term groundwater level increases are not expected to cause basement flooding

Questions