Hydrologic Connectivity of Isolated Wetlands Todd C. Rasmussen, Ph.D. Associate Professor of Hydrology Warnell School of Forest Resources University of.

Hydrologic Connectivityof Isolated Wetlands

Todd C. Rasmussen, Ph.D.Associate Professor of Hydrology

Warnell School of Forest Resources

University of Georgia, Athens GA 30602-2152

www.hydrology.uga.edu

Abstract

• Recent changes in the judicial interpretation of isolated wetlands has caused the Savannah District of the U.S. Army Corps of Engineers to consider removing these wetlands from their jurisdiction.

• If this policy is adopted, many coastal freshwater wetlands will be threatened because of their isolation from other waterbodies by surface water connections.

• This paper examines the degree of subsurface hydrologic connection between shallow depressional coastal wetlands with other jurisdictional waterbodies.

Abstract (cont.)

• It is shown that shallow groundwater flow between isolated wetlands and jurisdictional waterbodies occurs for a wide range of surficial aquifer conditions.

• The magnitude of hydraulic linkage is a function of

– the properties of the surficial aquifer,

– the properties of the wetland,

– the distance between the wetland and the jurisdictional limit.

• Hydrologic analyses should be conducted prior to the removal of isolated wetlands to confirm their lack of subsurface connectivity with nearby waterbodies

Background• For the first thirty years of its history, the Clean Water Act was

interpreted in a manner that afforded protection to nearly all waters and wetlands, including so-called isolated wetlands.

• In 2001, however, the U.S. Supreme Court issued a decision in Solid Waste Agency of Northern Cook County v. U.S. Corps of Engineers (the SWANCC decision) that called into question the federal government's ability to regulate isolated waters.

• Despite the growing trend to interpret the SWANCC decision narrowly, the Savannah District of the U.S. Corps of Engineers appears intent on using the decision as a basis for severely limiting federal protections for freshwater wetlands in Georgia.

Background (cont.)

• The Corps appears unwilling to regulate wetlands unless a continuous surface water connection exists between the wetlands at issue and other jurisdictional waters.

• Because the state of Georgia does not protect freshwater wetlands, the unwillingness of the Corps to protect wetlands that it deems to be isolated is a significant problem that exposes thousands of acres of Georgia's wetlands to many threats, including but not limited to mining, silviculture, and commercial and residential development.

Approach

• This presentation examines the degree of interconnection with U.S. waters even though they lack explicit surface-water connections.

• Because these wetlands are located in coastal areas where subsurface hydrologic flow can be significant, they may still be hydrologically interconnected with U.S. waters due to subsurface flow.

• Various subsurface factors are evaluated in determining the magnitude of the interconnections, including– the physical and hydraulic properties of the aquifer and the wetlands, and

– the distance between the wetlands and U.S. waters

Modeling Approach

• A two-dimensional (profile), steady flow domain.

• Saturated ground-water flow within the flow domain.

• The thickness of the aquifer varies

• The distance from the wetland to U.S. waters varies

• The depth of the wetland varies

• The depth of the waterbody varies

• The extent of the wetland varies

Problem Geometry

Assumptions

• Homogeneous: no variation in position

• Isotropic: no variation in direction

• Saturated: flow below the water table

• Finite Extent: limited zone of influence

• Two-dimensional: longitudinal features

• Steady Flow: no aquifer storage

CVBEM Complex Variable Boundary Element Method

• Mathematical Model that uses complex potential, w = h + i s– h is the hydraulic head, s is the streamline, i = sqrt (-1)

• Uses Cauchy Integral Theorem to model flow within the domain• Requires specification of

– domain size– material properties (hydraulic conductivity)– boundary conditions

• Uses Ordinary Least Squares (OLS) to solve the resulting over-determined system of linear equations

• Provides estimates of total head, streamlines, and fluxes within the flow domain

Cauchy Integral

Internal to domain:

Boundary:

Hodograph

Base Case: Other scenarios are compared to these conditions

• Properties:– Aquifer thickness: b = 20 m

– Separation distance: L = 60 m

– Wetland depth: d = 1 m

– Waterbody depth, D = 1 m

– Wetland extent: w = 10 m

– Waterbody extent: W = 20 m

– Elevation difference: h = 20 cm

Changes from Base Case

• Aquifer permeability:– Doubling permeability increased flow by 100%

– Halving permeability decreased flow by 50%

• Elevation difference:– Doubling elevation increased flow by 100%

– Halving elevation decreased flow by 50%

Changes from Base Case (cont.)

• Aquifer thickness:– Doubling thickness increased flow by 74%

– Halving thickness decreased flow by 47%

• Separation distance:– Halving distance increased flow by 78%

– Doubling distance decreased flow by 47%

Changes from Base Case (cont)

• Waterbody depth:– Doubling waterbody depth increased flow by 0%

– Halving waterbody depth decreased flow by 0%

• Wetland depth:– Doubling wetland depth increased flow by 1.5%

– Halving wetland depth decreased flow by 0.5%

• Wetland extent– Doubling wetland extent increased flow by 17%

– Halving wetland extent decreased flow by 7%

Limitations

• Fails to account for dynamic conditions– Short term effects may be even greater

• Fails to account for anisotropy– Effective aquifer thickness will be affected

• Fails to account for limited longitudinal extent– Circular features may be affected to a greater degree

• Overall, effects on wetlands are underestimated.

Conclusions

• Small, isolated wetlands can interact with coastal waters through the subsurface (i.e., ground water)

• Even small wetlands at large distances from the coast are part of the coastal hydrologic continuum

• Protecting small, isolated coastal wetlands should be considered.

Recommendations• Prior to removal, isolated wetlands should be

evaluated to assure that they are not actively contributing to the coastal hydrologic system

• At a minimum, required studies should include the determination of:– The response to water level changes in nearby

waterbodies– Aquifer properties such as hydraulic conductivity,

anisotropy, and thickness– Wetland and nearby waterbody characteristics, such as

depth, extent, etc.– Computer modeling to determine the quantity of

subsurface flow that the isolated wetland contributes the regional hydrology

Acknowledgement

• I extend special thanks to Chris DeScherer, a staff attorney with the Southern Environmental Law Center in Atlanta, GA, for bringing this issue to my attention