Hydrologic Connectivity of Isolated Wetlands

1. 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

3. 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.

4. 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

5. 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.

6. 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.

7. 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

8. 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

9. Problem Geometry

10. 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

11. CVBEMComplex 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

12. Cauchy Integral Internal to domain: Boundary:

14. Hodograph

15. 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

16. 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%

17. 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%

18. 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%

19. 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.

20. 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.

21. 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

22. 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