Ground-Water Mounding

START DATE: 15-MAY-2007 

END DATE: 30-SEP-2009 

PROJECT NUMBER: 2454D1R 

STUDY OBJECTIVES:  

The objectives of this project are to: 

1. Determine the hydrogeologic properties that are relevant in controlling the magnitude and extent of ground-waterlevel rise (mounding) resulting from stormwater infiltration/recharge facilities, and to determine a range of values for these properties that would be expected in shallow aquifers typical of New Jersey; 
2. Investigate the sensitivity of the magnitude and extent of ground-water-level rise (mounding) resulting from stormwater infiltration/recharge facilities to the variation in relevant hydrogeologic properties. 
3. Summarize the sensitivity with graphs, nomographs, etc. to enable designers of infiltration facilities to evaluate the impact of their design on the ground-water table and the extent (lateral and vertical) of possible effects. 
4. Evaluate the analytical limitations and uncertainties to facilitate the establishment of safety factors for the use of the results. 
5. Prepare a report summarizing the approach, detailing specific findings, and clearly identifying the limitations of the analyses. The report will include an electronic copy of all model input and output data used for the analysis. The report will be public-domain information, which will facilitate the easy use of the findings by the State of New Jersey. 

STATEMENT OF PROBLEM:  

Stormwater management rules set a mandatory state-wide standard to maintain 100% of the existing average annual recharge volume in a site from pre- to post-development. In addition, it has been widely recognized that the infiltration of stormwater runoff is an effective means to address increases in runoff volume. Potential adverse impacts include, but are not limited to, exacerbating a naturally or seasonally high water table so as to cause surficial ponding, flooding of basements, or interference with the proper operation or subsurface sewage disposal systems and other subsurface structures in the vicinity or down gradient of the groundwater recharge area 

STRATEGY AND APPROACH:  

The project objectives will be accomplished using an approach where the volume and timing of the infiltrating recharge to the shallow ground-water system (water table) from a stormwater infiltration structure will be specified in a series of simulations using the 3-dimensional ground-water flow model, Modflow (McDonald and Harbaugh, 1988). The sensitivity of the magnitude, extent, and timing of the resulting ground-water mounding will be examined using these simulations where the model parameters representing relevant hydrogeologic properties are varied through a range of values that is expected for conditions typical of New Jersey, including the Coastal Plain, Piedmont, and Highland terrains. The “outer envelop” of the sensitivity analysis will be used to define the conditions where mounding may lead to problems. The types of hydrogeologic properties that could be tested are: aquifer hydraulic conductivity and specific yield, basin relief, structure freeboard (ambient height above the water table), presence and depth to the first significant confining bed, aquifer stratification in the near subsurface, and direction of preferential permeability (especially in fractured rock terrain), to name a few. For the sake of simplicity, the input storm sequence, infiltration structure area, and consequential ground-water recharge volume would be the same in all runs. 

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