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Home My WebLink AboutSDP201800046 Assessment - Groundwater 2018-08-01 sod Mint Springs Park41\q-j-ti Maintenance building Tier 3 Groundwater Assessment vs Groundwater Management Plan 7 Prepared for: B. Clark Gathright, LLC Charlottesville, VA Draft submitted August 1, 2018 Nick H. Evans PhD CPG Virginia Groundwater LLC PO Box 1424 Charlottesville VA 22902 nick(@virginiagroundwater.com 434-466-1280 1 Key Findings Hydrogeologic setting: Located in steep topography on the eastern flank of Bucks Elbow Mountain, the site is underlain by a thick deposit of colluvium over fractured crystalline charnockite bedrock. Groundwater availability: Recharge potential to site is excellent. Significant quantities of groundwater flow through colluvium, supplying recharge to fractures in underlying bedrock. Wells tapping into bedrock fracture networks beneath colluvium have potential for significant sustainable yields. Are Hydrogeologic conditions favorable to proposed use? Yes. Contamination threats on record within 2000 feet of parcel? Yes. Additional contaminant threats observed in field reconnaissance? None. Anticipated impacts of proposed use on existing users of groundwater: None. Groundwater management plan: Offset groundwater recharge lost due to impervious surface created during development by infiltrating runoff through 3.75 acres of permanently protected SWM forest and open space land. PNI Project Overview The proposal is to construct a new maintenance building on 0.87 partially cleared acres at Mint Springs Park, about 2 miles northwest of Crozet in western Albemarle County (Figure 1). Figure 1 : Site location ( ® ) in western Albemarle County The proposal includes a 2,068 square -foot building with attached 2,152 square - foot open shed roof and gravel parking area. As part of the storm water management strategy for the project, an additional 3.75 acres are to be protected from future development as SWM forest and open space land (Figure 2). The project is entirely within Mint Springs Park (TMP 55-29, 455 acres); the northern boundary of the project site is on the northeastern property line of the Park. A site plan showing the project layout and area of land disturbance is provided with this report. 3 To the west and southwest of the project site, mixed hardwood forest cover extends to and beyond the Park boundary, a distance of more than a mile. Open Park land and water features are within 1000 feet of the site, to the south and east. Parcels to the immediate north are in residential use on 2 -acre lots. The closest residence is about 800 feet north of the site. Projected maximum water consumption for the facility is 100 gallons per day, based on a small number of employees expected to be using restroom facilities. Water will be supplied by a well to be drilled on site. Waste water will be disposed of through an on-site drain field. Figure 2: Location map (detail) showing proposed maintenance facility footprint, land designated for SWM forest & open space, Mint Springs Park boundary, and nearby parcel boundaries 2 ►J maintenance facility footprint i SWM forest and open space land Hydrogeologic Assessment Topography and hydrology The site is located at about 1035 feet elevation within the Powells Creek drainage (Figure 3), on the southeast flank of Bucks Elbow Mountain (elevation 3165 feet). Powells Creek feeds into the Beaver Creek — Mechums River drainage, within the upper part of the South Fork Rivanna River drainage basin. The crest of Bucks Elbow Mountain is about 2 miles northwest of Mint Springs Park, which means there's an average topographic gradient of about 1000 feet per mile north and west of the project site. This steep topography sets up potential for strong hydraulic gradients on surface streams, and in groundwater flow. The groundwater flow direction beneath the project site is predicted to be southeast, or down the projected hydraulic gradient. Figure I Topography, drainage divides and predicted groundwater flow paths in the vicinity of Mint Springs Park proposed maintenance facility drainage divide predicted groundwater flow direction maintenance facility site Hydrogeology Field mapping indicates that project site is entirely underlain by a layer of colluvium 25 or more feet thick, that occurs on top of crystalline bedrock (Figure 4). Colluvium is a porous mixture of rocks and soil that have been transported down slope by the action of gravity and moving water. The colluvium beneath the site is within an extensive deposit that underlies much of the northern portion of Mint Springs Park and extends northward beneath residential parcels beyond the Park boundary. Colluvium is a very good medium for absorbing rainwater into the ground, recharging groundwater, and enabling groundwater to flow down the hydraulic gradient beneath the land surface. The Unison and Thurmont soils Figure 4: Geology in the vicinity of Mint Springs Park 1000 feet=ti 9.i�y.Nf I colluvial material at the surface (thickness greater than one meter) bedrock at or close to surface (charnockite, greenstone and metasandstone, undivided) I on site, which are derived from colluvium, are described in the USDA Soil Survey as deep and well -drained, with high water storage capacity and capacity to transmit water. These are attributes of excellent groundwater storage and transmission medium. Crystalline bedrock (charnockite beneath the project site, greenstone or metasandstone elsewhere in the vicinity) is at or close to the surface at higher elevations to the west and south of the project site, but up to several tens of feet beneath colluvium at the site. Crystalline bedrock does not contain primary intergranular porosity through which groundwater might flow. Rather, groundwater flows in these rocks within fractures and fissures where such occur. The colluvial deposits on top of the bedrock capture significant quantities of rainwater, which is then available to recharge fractures and fissures where they occur in the underlying bedrock. Suitabilitv of site for water well construction In this geologic setting, drinking water wells are normally constructed to tap water from bedrock fractures, with sufficient casing to exclude potentially contaminated water that may exist in near -surface colluvial deposits. Groundwater becomes cleansed by passing through deeper colluvium and bedrock fissures. While the colluvium provides an excellent medium for recharge and groundwater storage, a successful well must intersect deeper water -bearing fracture networks in the bedrock that are interconnected with the colluvium recharge source. The reported yields from drilled wells are a good proxy for degree of bedrock fracture density. Of eleven database wells within 2000 feet of the site (Figure 5) for which yields are available in the County database, the average is 23.8 gallons per minute (GPM), with a low of 1 GPM and a high of 66 GPM. These numbers imply that overall bedrock fracture density is favorable to water well productivity in the vicinity of the project site. The combination of favorable recharge and bedrock fracture density indicate that conditions on the project site are favorable for drilling a well capable of meeting the proposed use of 100 gallons per day. Potential for proposed use to affect existing users of groundwater There are several residences on adjoining parcels that rely on individual domestic wells for water supply (Figure 5). The closest of these, on TMP 39-21f in the Emerald Ridge subdivision, is approximately 800 feet from the project site. These private wells are all up topographic and hydrologic gradients from the proposed project site. A well on the proposed site will almost certainly have no impact on groundwater availability or quality for any of these wells because: 7 Figure 5: existing wells and reported yields (GPM) as available in County database within 2000 feet of proposed maintenance facility 1. Recharge to the wells on adjoining parcels north of the Park is estimated to come from farther up drainage (to the north and west; Figure 3), rather than from the vicinity of the proposed site, which is to the south; and 2. The estimated peak groundwater usage of 100 gallons per day at the proposed site is very modest relative to available groundwater, and is not consumptive, to the extent that much of the water will be returned to the ground as recharge via the drain field. According to the Albemarle County Parks and Recreation Department there are two wells currently serving the needs of the Park (Figure 5). The more northeastern of these is about 700 feet south of the existing maintenance building, supplying that facility. The other well is located close to the bath house at the swimming area, supplying that facility. Each of these wells is down • domestic well municipal well (not in service) 0 current Mint Springs Park supply well gradient from the proposed project site. However, it appears highly unlikely that the new facility would impact groundwater availability for either well, given the estimated modest peak groundwater usage of 100 gallons per day at the proposed site. There are several municipal wells, part of the former water supply for the Town of Crozet, within 2000 feet of the parcel. None of these are currently being pumped, and a spokesperson for the Albemarle Service Authority reports there are no plans to reactivate these in the future. There is no anticipated impact to these wells from the proposed project. Contaminant threats affecting the proposed site There are three documented petroleum releases within1000 feet of the proposed project site (Figure 6). Each of these is listed as a "closed case" in the Virginia DEQ database, meaning that mitigation actions have been completed to State and Federal specifications. These sites are all down topographic and hydraulic Figure 6: Predicted groundwater flow directions in the vicinity of proposed maintenance facility, and contamination sites on record predicted groundwater flow direction contamination site on record gradient from the site. None poses a potential threat to groundwater on the proposed project site. There are several existing residential drain fields up hydraulic gradient and within 1500 feet of the site. If one of these were to fail, the effluent would flow down gradient into Mad Run as surface runoff and not reach groundwater on the project site. Threats of contamination by proposed project The principal threats of groundwater contamination by the proposed facility are in the form of potential contaminant spills and runoff (petroleum, fertilizers, pesticides) that occur post development during vehicle / equipment servicing. If a hypothetical contaminant spill were to occur at the proposed facility, any resulting plume significant enough to affect local groundwater would move down gradient (southeast), away from the Emerald Ridge subdivision and toward the existing Mint Springs wells. Reserve wellfield There is ample space on site for a replacement well in the event that the primary well fails due to contamination, lack of water or other issues . Water budget analysis of project impact It is instructive to use quantitative analysis to evaluate the impact to groundwater recharge due to creation of impervious surface as result of site development. Existing natural recharge to the entire site (0.87 acres developed site plus 3.75 acres SWM forest and open space land) can be modelled as follows: Annual regional precipitation: 43 inches Conservative estimate for the percentage of precipitation contributing to groundwater recharge, subtracting runoff and evapotranspiration: 15% Annual regional groundwater recharge: 6.45 inches Average regional daily groundwater recharge- .0179 inches = .0015 feet Daily recharge per acre: .0015 feet X 43560 square feet per acre = 65.6 cubic feet recharge per acre Gallons recharge per day per acre: 65.6 cubic feet X 7.48 gallons per cubic foot = 491 gallons per day per acre Gallons per day natural recharge to site: 491 gallons per acre X 4.62 acres = 2,268 gallons per day. 10 It has been calculated as part of the project Water Quality Plan (attached) that the project will result in a net increase of impervious surface of 0.67 acres. The impact on post -development groundwater recharge is estimated below: Annual regional precipitation: 43 inches Conservative estimate for the percentage of precipitation contributing to groundwater recharge, subtracting runoff and evapotranspiration: 15% Annual regional groundwater recharge: 6.45 inches Average regional daily groundwater recharge: .0179 inches = .0015 feet Daily recharge per acre: .0015 feet X 43560 square feet per acre = 65.6 cubic feet recharge per acre Gallons recharge per day per acre: 65.6 cubic feet X 7.48 gallons per cubic foot = 491 gallons per day per acre Gallons per day recharge lost due to new impervious cover: 491 gallons per acre X 0.67 acres = 329 gallons per day. To some extent, the loss of natural recharge due to creation of impervious cover will be mitigated by the infiltration of runoff from the developed portion of the site, onto the project's 3.75 -acre SWM forest and open space lands. However, there will be losses due to overall runoff and evapotranspiration such that recharge mitigation is not likely to be 100% efficient. Relative to the proposed usage of groundwater post development, overall recharge post -development will be more than adequate to supply groundwater usage without long-term degradation of groundwater resources: Predicted maximum daily groundwater withdrawal on site: 100 gallons per day Estimated proportion of daily withdrawal that is consumptive (water not returned to the ground on site through the drain field, for example water carried off-site in containers, or water lost to evaporation): 5%, or 5 gallons per day Groundwater management plan Offset groundwater recharge lost due to impervious surface created during development by infiltrating runoff to the extent possible within the 3.75 acres of permanently protected SWM forest and open space land. 11 Submitted by Nicholas H. Evans, CPG # 2801 001041 July 31, 2018 CO-MINIONTWEALTH of VIRGINIA Department of Professional and Occupational Regulation EXPWIES ON NUMBER LO $- 9960 Mayland Drive, Suite 400, Richmond. VA ?3233 31-2019 Telephone: (804) 367-8500 ` 2801001041 BOARD FOR PROFFSSIONAL SOIL SCIENTISTS, WETLAND PROFESSIONALS & GEOLOGISTS CERTIFIED PROFESSIONAL GEOLOGIST �j NICHOLAS H EVANS .] �,'; 4809 BURNLEY STATION ROAD rev a BARBOURSVILLE, VA 22923 -rFY Status ran be verified at http91vvvvw.dpar.vtrginia.guv 12