Presentation on theme: "Groundwater Recharge and Groundwater Quality Issues Vincent W. Uhl, PH, PG Vincent Uhl Associates, Inc. Lambertville, NJ Slides 11 to 23."— Presentation transcript:
Groundwater Recharge and Groundwater Quality Issues Vincent W. Uhl, PH, PG Vincent Uhl Associates, Inc. Lambertville, NJ Slides 11 to 23
Groundwater Recharge as a basis for planning Geology dependent – Diabase/Lockatong compared to Brunswick and Pre-Cambrian rock aquifers Use Average or extreme recharge events ? Local experiences in the recent “Drought” ? Are water use estimates used in developing lot sizes realistic ? Aquifer Storage as a Drought Buffer E.g. 1 acre is underlain by 1 million gallons of groundwater in storage at a porosity of 1%. With that storage, a 3-acre lot could supply a family with water for over 30 years absent any recharge.
Enter Water Quality Nitrate is the water quality indicator that has been used to date. The drinking water standard for nitrate is 10 milligrams per liter (mg/l). Anti-degradation limits for nitrate to protect surface water systems is in the range of 6 mg/l. Water professionals have been using nitrate-dilution models developed 25 years ago. Should other chemicals be considered in planning and more importantly in protection measures?
Nitrate Dilution – the Math Basically a mixing model. Recharge water is mixed with septic effluent to yield a mix with a nitrate concentration below some criteria (the drinking water standard or anti-degradation limit for nitrate). Dilution in the aquifer system is not taken into account in the models. Nitrate renovation in the unsaturated zone is sometimes not taken into account.
Nitrate Dilution – the Equation A = 640RCeQeP / IC 1 A = Average Area per dwelling unit in acres R = nitrate renovation factor = 0.80 Ce = Nitrate input from septic leach field = 40 mg/l Qe = Per capita input to septic system in gpd/person P = Number of people per dwelling unit. I = Natural recharge rate in gallons per day per square mile C 1 = Acceptable nitrate concentration – Drinking water standard at 10 mg/l or anti-degradation limit of say 6 mg/l.
Calculation Example A = 640RCeQeP / IC 1 R = 0.80 Ce = 40 mg/l Qe = 80 gpd/person P = 4 C 1 = 6 mg/l Using the recharge rates provided above of: 85,000 gpd/mi 2 (130 gpd/acre); 225,000 gpd/mi 2 (315 gpd/acre); 525,000 gpd/mi 2 (820 gpd/acre) This translates to the following lot sizes: For recharge rate of 130 gpd/acre: A = 13 acres For recharge rate of 315 gpd/acre A = 5 acres For recharge rate of 820 gpd/acre A = 2 acres
Should this be the basis for planning? End result to date has been fairly large lot sizes. Basis = an equation which might need some field verification. Field based nitrate studies would be helpful to assess nitrate concentrations: Immediately beneath/downgradient from a septic leach field. Over different lot sizes Over varying geology
Alternatives to Consider Use recharge estimates on a macro scale – as they should be viewed - to evaluate options such as cluster development with open space. Use Hydrogeologic professionals in a role to best determine how to configure development and not just for pumping test programs. In the alternative, hydrogeologists could be used to: Look at how to configure a development from a water resource perspective. Assess optimal well and septic field locations Assess optimal areas for open space (groundwater recharge opportunity). Protect stream and drainage corridors. Optimize storm water and paved surfaces runoff management.
Other Alternatives Community septic systems for clustered development. Small wastewater treatment plants. Connect to regional or local sewage treatment plant. Gray water reuse.
Local Planning Challenges Storm water management so as to enhance groundwater recharge as well as to maintain stream baseflow and stream corridor protection. Educational programs in regard to household products with harmful chemicals that end up in septic systems and ultimately in groundwater. Promotion of groundwater recharge from paved and roofed surfaces: Roof drains Vegetated swales Porous pavement and sidewalks Etc.
This diagram shows how development and its corresponding increase in impervious cover disrupts the natural water balance. In the post-development setting, the amount of water running off the site is dramatically increased. Center for Watershed Protection “Impacts of Urbanization”
Limitations Low yielding aquifers in certain areas – Diabase and Lockatong. Natural groundwater quality constituents – e.g. Arsenic. Human-induced groundwater quality impacts. Reduction of baseflow.
Opportunities Use knowledge of groundwater resources to look at: Development alternatives. Optimal areas for open space consideration and to preserve. Optimal areas for wells and septic systems. Creative ways to recharge groundwater.
Thank You Vince Uhl Return to Hunterdon County Smart Growth