Jul 22, 2013 - to the township who would be at risk of having to providing ... Quality of Northern Bucks County, Pennsylvania USGS 94-4109 RA Sloto & CL.
Recharge Threshold Fractured Bed Rock Geology R.B. Stanfield Stanfield Associates July 22, 2013 The use of bed rock geology as a predictor of recharge has been a conventional wisdom. A closer examination of the literature and GWMC data shows that this is a weak assumption in fractured bed rock geologies. (FBRG) The wide variation of reported recharge estimates suggests there are more factors affecting this ground characteristic. Most particularly, groundwater recharge and storage are controlled in FBRG more by secondary fracture porosity rather than the intrinsic rock internal void spaces. The fracturing is influenced by the rock type, but it can vary widely. The Tinicum Ordinance 123 and the new template ordinance skirt this problem by using bed rock geology as • only a threshold lowest level withdrawal limit above which • SALDO applicants must demonstrate greater groundwater availability by aquifer testing and other measures. The choice of a lower recharge threshold limit has not been examined quantitatively. The question proposed is: What is a safe recharge estimate for a specific fractured bed rock geology below which would constitute an acceptable risk for a potential over withdrawal of groundwater from a applicant's SALDO project? This question is posed when a SALDO applicant applies for a project permit. The EAC, Planning Commission and Board of Supervisors must decide whether to allow the applicant to proceed using a simpler hydrogeologist's report using a geological recharge based estimate vs requiring an extensive and expensive aquifer test and analysis. The decision must be based on • •
a risk of approving a project that might over estimate the capacity of the underlying aquifer to provide the estimated demand vs the cost to the developer for proving the capacity.
The risk of over estimating the capacity • to the developer would be the build out cost of the project only to discover that there is not enough water making the project economically viable, • to the ultimate owners who lose the value of their investment because they are left high and dry, and • to the township who would be at risk of having to providing municipal water and sewage utilities. The following is calculation to estimate of a threshold recharge that assures that 90% of projects that 1
have groundwater withdrawals from the identified fractured bed rock aquifers less the computed figure will not exceed the average recharge with a 67% drought safety factor. Conclusions Safe threshold recharge Bed Rock Geology diabase Lockatong Brunswick
gpd/Ac 51 84 107
Note: 1. The storage capacity is in the fractures rather than in the porous rock structure. The recharge will be largely controlled by the vertical Lockatong and diabase fractures. The storage capacity will be largely controlled by the fracture voids in the Brunswick geology. 2. The variation in recharge from similar FBRG vary by 6-10 fold. 3. In choosing a set of data for analysis, the data must be from very similar rain/snow fall patterns. 4. The top bed rock geology does not necessarily characterize the club sandwich of rock layers that extend through the aquifer zones See Fig 1. 5. Most wells in the BNT derive their water from moderately deep (>50'→ 250') semi-confined aquifers1 below the top bed rock zone. 6. Most well yield comes from defined vertical zones in the well that are fractured. 7. Confined aquifers2 are not common3. 8. FBRG in the area are typically small with apparent horizontal extents of a few hundred yards to low single digit miles. A number of aquifers appear to be bounded4. 9. In addition to precipitation, some aquifers are fed by losing water bodies5.
1 2
3 4
5
An aquifer partially confined by soil layers of low permeability through which recharge and discharge can still occur. An aquifer that is sandwiched between two aquitards. Confinement can raise the water pressure in an aquifer sufficiently high to produce an artesian well. See Smithtown below Cafferty. A bounded aquifer is one that has side impermeable boundaries that have a limited capacity measured in gal/foot of vertical extent. Analyzing Pumping Test Data from a Bounded Aquifer RBStanfield, Feb. 15, 2009 Hydrogeological and Groundwater Quality of Northern Bucks County, Pennsylvania USGS 94-4109 RA Sloto & CL Schreffler p53
Mean gpd/mi2 Mean gpd/Ac Ac/4 BR house 1σ gpd/mi2 90% Uncertainty range Lo 90% Uncertainty range Hi 10%ile gpd/ac Threshold Recharge 90% Uncertainty range gpd/Ac
where: gpd/mi^2 gpd/Ac Ac/4 BR House 90% percent rage Lo 90% percent rage Hi Threshold Recharge
gallons per day per square mile, gallons per day per Acre, Number of Acres required for a 4 Bedroom House assuming 500 gpd consumption and an average recharge with a 67% drought Safety Factor, Lower bound of the 90% range; the 5th %ile, Upper bound of the 90% range; the 95th %ile, The 10th %ile with a safety factor (67%).
Note: Both arithmetic and geometric statistics are presented. Because of the very wide range of the data, the geometric metric is preferred. These threshold recharge estimates should be used in the template ordinance as the boundary between requiring an aquifer test or not.
Mean gpd/mi2 Mean gpd/Ac Ac/4 BR house 1σ gpd/mi2 90% Uncertainty range Lo 90% Uncertainty range Hi 10%ile gpd/ac Threshold Recharge 90% Uncertainty range gpd/Ac
Mean gpd/mi2 Mean gpd/Ac Ac/4 BR house 1σ gpd/mi2 90% Uncertainty range Lo 90% Uncertainty range Hi 10%ile gpd/ac Threshold Recharge 90% Uncertainty range gpd/Ac
5
Figure 1 Typical Fractured Bed Rock Geology Hanson Quarry Illustrating the layering of Lockatong (solid layers) and Brunswick (rubbly players). The recharge will be largely controlled by the vertical Lockatong fractures. The storage capacity will be largely controlled by the fracture void in the Brunswick.
