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Home My WebLink AboutSDP201600014 Assessment - Groundwater 2016-05-26 (2)Wildrock TMP 6-28B Tier 3 Groundwater Assessment Groundwater Management Plan (DRAFT) Prepared for: Shimp Engineering, P.C. Charlottesville, VA April 29, 2016 Nick H. Evans PhD CPG Virginia Groundwater LLC PO Box 1424 Charlottesville VA 22902 Key Findings Hydrogeologic setting: Located in steep topography on the eastern flank of the Blue Ridge, the property is underlain by fractured charnockite bedrock and thick deposits of colluvium. Groundwater availability: Recharge potential to site is excellent. Significant quantities of groundwater flow through bedrock fracture networks and overlying colluvium. Wells tapping into bedrock fracture networks beneath colluvium have potential for significant sustainable yields. A 60+ gallon per minute well has recently been drilled on the parcel. Are Hydrogeologic conditions favorable to proposed use? Yes. Contamination threats on record within 2000 feet of parcel? None. Additional contaminant threats observed in field reconnaissance? None. Anticipated impacts of proposed use on existing users of groundwater: None. Groundwater management plan: Protect and expand where possible the amount of forest cover on the property during construction and beyond. Implement runoff -neutral development as specified in the development plan, to include infiltration through a rain garden, and by on-site waste disposal through a sanitary drainfield. Ensure long-term viability of the rain garden facility by performing maintenance as needed. Project Overview The proposed development is located on a 28.01 acre parcel in northern Albemarle (Figure 1), off Rt. 810 about 5 miles northeast of White Hall. The proposed use is a day camp with 6 primitive overnight camping sites and an environmental education center. Figure 1 H Little Flat Mountain FafrMwAen f1ram ref" Prtlsal" AAemarle GIS; ShirM EngmeerxV The parcel is located on two unnamed tributaries of Buck Mountain Creek, which flows into the South Fork Rivanna River upstream from the South Fork Rivanna Reservoir. The parcel is on the south flank of Little Flat Mountain, which has a summit elevation, in Shenandoah National Park, of 3125 feet. Elevations on the parcel range between about 950 feet in the southeast corner to 1200 feet in the northern corner (Figure 2). Figure 2: TMP 6-28B topography (4 -foot contour interval) and surface water features (blue) Albema a GIS: Nick Evan 04/29/2016 Existing land cover on the parcel is open pasture at lower elevations, on the southeastern portion, and mixed second -growth forest on the remainder. A site plan showing the existing buildings, proposed development layout and approximate land disturbance is attached to this report. Projected maximum water consumption for the facility, based on the VDH- approved on-site wastewater treatment design capacity, is 765 gallons per day. A well was recently drilled on the property with a blown yield in excess of 60 gallons per minute (personal communication, Foster Well Company). Surrounding parcels are primarily forested. There are residences on three adjoining parcels to the west and south. To the north the parcel adjoins the Patricia Ann Byrom Forest Preserve, owned by Albemarle County and developed as a park with trails open to hiking and equestrian use. Hydrogeologic Assessment Geology Field mapping indicates that bedrock beneath the parcel is a very coarse-grained granite, geologically termed charnockite. This bedrock is exposed at the surface in many places in uplands, in the southwest and northern portions of the parcel. At lower elevations, in the central and eastern portions, bedrock is largely covered by a thick mantle of colluvium, which is a mixture of rocks and soil that has been transported down slope by the action of gravity and moving water (Figure 3). Figure 3: Site geology showing distribution of charnockite (ch) and major colluvial deposits ( ) on the parcel Albemale G1S; Mick Evan 04/2912016 Groundwater recharge and flow paths Recharge for groundwater beneath the parcel occurs both within the parcel and on about 740 acres of forested land that is topographically up -gradient, to the north and west, on the south flank of Little Flat Mountain. The entire parcel is itself a recharge area for groundwater flowing down -gradient to the northeast, Figure 4: Predicted groundwater flow paths (-� and groundwater divides ( ) referred to in text Albemarle GIS; Nick Evans 04/29/2016 into the Buck Mountain Creek drainage. In general, groundwater flow follows hydraulic gradients that mirror topographic gradients (Figure 4). Bedrock fracture density and water well productivity Charnockite bedrock does not contain primary intergranular porosity through which groundwater might flow. Groundwater flow within the bedrock is confined to fractures and fissures that can serve as conduits for water. In contrast, colluvium is very porous, and serves as an ideal conduit for groundwater. Colluvial deposits on the flanks of Little Flat Mountain above this parcel, and on the parcel itself, are capable of capturing and transmitting as groundwater, significant quantities of rainfall. That water is then available to recharge fractures and fissures where they occur, within underlying bedrock. In this geologic setting, drinking water wells are normally constructed with sufficient casing to exclude water that exists within near -surface colluvial deposits. This is because colluvium is readily vulnerable to contaminants introduced at the surface. Hence water wells are drilled targeting bedrock fracture networks deep enough that recharge water from the overlying colluvium has had enough travel time and distance through the rock to be cleansed of contaminants. Successful water wells have both a reliable source of recharge (in this setting, colluvial material), and an open network of bedrock fractures that connect with the recharge source. TMP 6-26B has both of these ingredients. In addition to the mantel of colluvium, field observations indicate there are abundant north- northwest trending fractures present within the bedrock. The reported capacity of the well recently drilled on the property (60+ gallons per minute) is consistent with these conditions, and far exceeds the needs of this project. Water budget estimate for site It is instructive to review the proposed use of water relative to the amount of water available to the site from natural recharge. Annual regional precipitation: 44 inches Conservative estimate for the percentage of precipitation contributing to groundwater recharge, subtracting runoff and evapotranspiration: 15% Annual regional groundwater recharge: 6.6 inches Average regional daily groundwater recharge: .0181 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 on parcel: 491 gallons per acre X 28.01 acres = 13,753 gallons per day Gallons per day recharge to the parcel plus up -gradient forested acreage off -parcel to the north: 491 gallons per acre X 735 acres = 360,885 gallons per day Predicted maximum daily groundwater withdrawal on site: 765 gallons* *a significant portion of daily withdrawal to be returned as recharge via an onsite drainfield These numbers indicate that the proposed use of groundwater is negligible relative to the abundance of groundwater that naturally occurs on the parcel. Potential for proposed use to affect existing users of groundwater There are 3 residences on adjoining parcels that rely on their own water wells (Figure 5). TMP 6-28A and TMP 6-35 have reported yields of 2 and 3 gallons per minute, respectively. No records were found for a well on TMP 6-35A. It is not anticipated that the proposed use on TMP 6-28B will impact any of the wells on adjoining parcels or elsewhere, for the following reasons: Figure 5: Existing wells and drainfields • (locations approximate) within 1000 feet of PAP 6-28B Albmmerk G13; Nick Evan 04/29/2016 1) Recharge to each of the wells on adjoining parcels is estimated to come from a different source from that on TMP 6-28B. In the case of TMP 6- 28A, it appears the well is receiving recharge primarily from the south and west, southwest of a groundwater divide between 6-28A and 6-28B (Figure 4). In the case of 6-35 and 6-35A, both parcels are southeast of the principal northeast trending drainage (a groundwater divide) that parallels Rt. 810 and crosses the eastern margin of 6-28B. Both 6-35 and 6-35A receive recharge from the south (away from 6-28B; Figure 4). 2) The proposed peak groundwater usage of 735 gallons per day is very modest relative to available groundwater. Furthermore, this proposed withdrawal is not consumptive, to the extent that much of the water will be returned to the ground as recharge via the drainfield. It is not anticipated that this withdrawal will impact existing groundwater users on adjoining parcels or elsewhere. There are no public water supply wells within 2000 feet of the parcel. Contaminant threats There are no reported contaminant spills or other documented threats to groundwater in current County, State and Federal databases, within 2000 feet of the TMP 6-28B. There are three existing drainfields inferred within 1000 feet of TMP 6-28B (Figure 5). None of these is up gradient from the parcel; none poses a threat to groundwater quality on the parcel. The proposed use as a day camp with primitive campsites and education center does not involve activities that pose a threat of toxic spill under normal circumstances. Reserve wellfield Due to the size and hydrogeologic setting of TMP 6-28B, there are ample favorable locations for a replacement well in the event that the primary well fails due to contamination, lack of water or other issues . Groundwater management plan Protect and expand where possible the amount of forest cover on the property during construction and beyond. Implement runoff -neutral development as specified in the development plan. Ensure long-term viability of the rain garden facility by performing maintenance as needed. Submitted by May 3, 2016 Nicholas H. Evans, CPG # 2801 001041