Albemarle County Hydrogeologic Assessment Summary Report

Transcription

1 Albemarle County Hydrogeologic Assessment Summary Report Phase II Groundwater Availability and Sensitivity Assessment with Proposed Groundwater Assessment Standards Presented To: County of Albemarle Department of Engineering and Public Works 401 McIntire Road, Room 211 Charlottesville, Virginia Prepared By: and County of Albemarle Department of Engineering and Public Works Staff December 30,

2 Table of Contents 1.0 Introduction Groundwater Availability Mapping and Classification Groundwater Sensitivity Mapping and Assessment Proposed Site-Level Groundwater Assessment Standards Recommendations for the Groundwater Management Program Available Information... 7 List of Tables Table 1 Table 2 Groundwater Sensitivity Buffers Development Criteria with Assigned Tier Rating List of Figures Figure 1 Figure 2 Albemarle County Hydrogeologic Unit Map Albemarle County Groundwater Sensitivity Map

3 1.0 Introduction This summary report provides an overview of Albemarle County s groundwater assessment efforts, particularly the phase 2 County-wide hydrogeologic assessment and the work of the Groundwater Committee. This work provides a technical foundation for the County to develop and implement groundwater policies needed to adopt the program. The County s groundwater work to date has focused on three technical areas: 1. As a long-standing Comprehensive Plan strategy, a County groundwater data base was initiated in 1998 as a pilot project for the Division of Mineral Resources (DMR). The aim of the data base was to collect historic well information from state agency records and put all the data into a GIS format. The County inherited the data base project from DMR in 2000 and completed the GIS for wells drilled through Under contract with the County, has conducted County-wide and watershed scale groundwater assessments. The phase 1 assessment (completed in April 2002) covered the Ivy Creek and Mechums River watersheds. Phase 2 completed assessment work for the entire County. The nature of this work was to explore and describe generalized groundwater conditions in the County, including the general availability and sensitivity of groundwater, utilizing the aforementioned data base, geologic and soil mapping, and other sources of information. These determinations are not designed to draw site-specific conclusions about groundwater, but to help focus what type of site-level groundwater analysis is warranted. 3. Simultaneous with ENSAT s work, the County s Groundwater Committee deliberated on the topic of appropriate site-level groundwater assessment standards for proposed development. This task was based on recommended action in the Comprehensive Plan. The Committee released its interim report, Underground Albemarle, in October Following several works session with the Planning Commission and Board of Supervisors, the committee continued to work with ENSAT to produce the recommended groundwater assessment standards, which were completed in July The County is now in a position to draw all of these efforts together into a more comprehensive groundwater program. This program would include: (1) adoption of groundwater assessment standards for proposed development, (2) development of a long-term groundwater monitoring program, and (3) the continuous updating and maintenance of the groundwater data base, relevant contaminant threats data bases (e.g., leaking underground tanks), and groundwater data from site, watershed, and County-wide studies. This summary report contains the following sections related to the phase 2 Groundwater Assessment project and the work of the Groundwater Committee: County-wide groundwater availability mapping and classification County-wide groundwater sensitivity mapping and assessment Recommended site-level groundwater assessment standards Recommendations for the groundwater management program 1

4 A detailed technical report has also been prepared that documents the project s methodology and results. The complete technical report is available from the Department of Engineering (see contact information at the end of the report). 2.0 Groundwater Availability Mapping and Classification The process of investigating groundwater availability at the County-wide scale involved identifying distinct hydrogeologic units and comparing the units based on groundwater availability characteristics for which data could be obtained. Each hydrogeologic unit shares some common characteristics with regard to geology, soils, and topography, although there is quite a bit of variability within each unit. Nine hydrogeologic units were identified across the County, as follows, and as shown on the Albemarle County Hydrogeologic Unit Map (Figure 1). 1. Blue Ridge West-IW: This unit lies primarily on the steep eastern slope of the Blue Ridge Mountain. Slopes range from 2-60%, and the soils are considered relatively deep and well drained. Rock types include volcanic greenstone and weathered granite and granite gneiss. 2. Blue Ridge East-IE: The Blue Ridge East unit overlies volcanic greenstone rock. The area is characterized by the presence of a prominent ridge that runs generally northeast within the otherwise rolling piedmont landscape. Portions of this ridge are known locally as Green Mountain, Carters Mountain, and the Southwest Mountains. 3. Colluvial Fans-II: The Colluvial Fan unit lies at the base of the eastern slopes of the Blue Ridge Mountain and is characterized by relatively steep drainage swales that extend to more gradual sloping conditions. Slopes range from 2-45%. Many of the soils in this area are transported from upslope areas through landslides and debris flows ( colluvial ) or the movement of water in drainage ways ( alluvial ). This is in contrast to most of the soils in the County, which are weathered from underlying bedrock ( residual ). 4. Piedmont Foothills-III: The Piedmont Foothills unit is characterized by rolling upland to relatively steep topography. Soils within this area are moderately deep, to deep, and generally well drained on the piedmont upland and foothills of the Blue Ridge Mountain with slopes ranging from 2-45%. Soils are formed from weathered granites and metagranites of the Blue Ridge Basement complex. 5. Lynchburg-IV: The Lynchburg unit is characterized by a narrow band that trends in a northeast direction and includes a relatively long segment of the Mechums River valley. Soils within this unit are predominantly deep to moderately deep and are well drained, with slopes ranging from 2-45%. The area is underlain primarily by the Lynchburg Formation, (or the Mechums River Formation) which consists largely of metagraywacke, meta sandstone, and graphitic schist. 6. Ragged Mountain-V: The Ragged Mountain unit is characterized by mountainous terrain located in the southwestern portion of the County and is underlain by the metagranitic rocks of the Blue Ridge Basement Complex. Slopes range from 2-60%. 2

5 7. Candler-VI: The Candler formation runs in a band near the eastern border of the County in a northeast direction. The rock types in the Candler (phyllites with some metasiltstone) tend to be less brittle and are generally more ductile than other rocks in the County, meaning that fractures can close up due to the weight of overlying material. Many of the soils are shallow and somewhat excessively drained. 8. Newark Basin-VII: The Newark Basin unit is characterized by low relief topography. The unit is underlain by the Newark Supergroup that consists of sandstones, siltstones and shales. Soils within this area are deep to moderately deep and well drained. Slopes range from 2-25%. 9. Piedmont Proper-VIII: The Piedmont Proper unit is characterized by gently rolling piedmont topography along the County s eastern border. The bedrock geology underlying the unit is mapped as Metagraywacke, quartoze schist and mélange. Soils range from shallow and somewhat excessively well drained to deep and well drained. Slopes range from 2-45%. Once the hydrogeologic units were delineated, the relative potential groundwater availability of each unit was assessed based on a model that incorporates the net effect of three interrelated variables: (1) thickness of overburden, defined as the soil and saprolite that overlie bedrock, (2) saprolite permeability, or the rate at which water can transmit through the saprolite, and (3) background fracturing, which describes the degree to which the bedrock contains water bearing features to feed wells. For each variable, the study team had to use available data from the County s groundwater data base, Albemarle County Soil Survey, and data derived from field investigations to approximate values and ratings for each hydrogeologic unit. For the purposes of this report, availability has been evaluated by reasoning associated with the following fundamental characteristics of hydrogeology in central Virginia s fractured bedrock aquifers. Precipitation infiltrates through soils and underlying saprolite (overburden) that overlie bedrock. The overburden acts in a manner similar to that of a sponge by receiving and temporarily storing water that has reached the depth of the local water table. The groundwater stored within the overburden feeds underlying fractures and secondary porosity features within bedrock that can transmit water vertically and horizontally. The borehole of a bedrock well penetrates water bearing features (fractures and secondary porosity features) within the bedrock which supply groundwater to the well. The thickness of the overburden beneath the groundwater table and the permeability characteristics associated with the overburden affect the availability of groundwater to underlying water bearing features within bedrock. Thicker saturated overburden possesses a greater potential for available groundwater storage than thinner saturated overburden. Permeability characteristics of the overburden affect the rate at which groundwater can move vertically or horizontally through the overburden. Higher permeability rates within the overburden media can potentially more readily provide groundwater supply to water bearing features than lower permeability media. Water bearing features in the bedrock (fractures) may be located directly below, in close proximity to, or thousands of feet away from the saprolite that is storing the water. The density, size, and interconnectivity of the fractures determine how much water is potentially available to a well. 3

6 Based on the results of the availability model, the nine hydrogeologic units were assigned a groundwater availability classification of 1, 2, or 3, with 3 being the highest relative availability (see Map 1). It must be noted that these results are generated at a broad scale, and are meant only for County-wide planning considerations and as a way to provide focus for site-level groundwater assessments. Groundwater availability at each site is heavily dependent on sitespecific features, so the availability classifications derived from this study cannot be used to draw conclusions at the site scale. 3.0 Groundwater Sensitivity Mapping and Assessment The purpose of the groundwater sensitivity assessment was to identify, at a planning scale, areas where there are red flags with regard to groundwater quality. For the purposes of this study, groundwater sensitivity was defined by the following two categories: Source Value identified areas that are sensitive because of their proximity to an existing valuable resource, such as a public water supply intake or existing community water supply well head. The intent here was that existing and future uses should not become a threat to valuable resources. For instance, septic drainfields associated with a new development should not endanger a nearby community water supply, or new wells should not interfere with the reliability of an existing community well. Contaminant Threats identified areas that are sensitive because of their proximity to an identified threat, such as a leaking underground storage tank. With these areas, the intention was to try to prevent newly-constructed wells from becoming contaminated. In the most dramatic case, which occurred several yeas ago in Albemarle County, a newly-constructed well became contaminated almost immediately from a known contaminant plume. Contaminant threats included in the analysis were leaking petroleum storage tanks (underground and above ground), landfills, and Superfund sites. The study team conducted an inventory of source value sites and contaminant threat sites available from local and state agencies, notably, the Department of Health, Department of Environmental Quality, and Albemarle County. Once all of these sites were identified and mapped, a fixed radius buffer was assigned to each category based on a literature review of appropriate buffers from state-level wellhead protection programs across the Country. These buffer areas are identified in Table 1 and shown on the Albemarle County Groundwater Sensitivity Map (Figure 2). TABLE 1 Groundwater Sensitivity Buffers Sensitivity Area Source Value Areas Public groundwater supply wells Public supply surface water intakes Contaminant Threat Areas Leaking petroleum storage tanks (underground & above ground) Landfills & Superfund sites Assigned Buffer ½ mile fixed radius ½ mile fixed radius (upgradient drainage area) 1,000 feet fixed radius ½ mile fixed radius 4

7 It is not intended that these sensitivity buffer areas serve as conclusive zones of known sensitivity. As stated, they serve as red flags (or planning scale information) for existing and proposed land uses within the buffers. The sensitivity zones were used to help construct the sitelevel groundwater assessment standards (see below) so that proposed development within these buffers could conduct more thorough analysis to help prevent future problems. Site-specific studies can more accurately determine the nature of proposed threats and how to prevent problems through design and management measures. 4.0 Proposed Site-Level Groundwater Assessment Standards As stated, the development of the site-level groundwater assessment standards was a collaborative effort between ENSAT and the County s Groundwater Committee. ENSAT produced draft standards that underwent revisions based on Committee comments. The Board of Supervisors authorized the formation and representation of the Groundwater Committee in May, 2000 based on strategies in the Comprehensive Plan as well as several work sessions with the Planning Commission. The purpose of the Groundwater Assessment Standards is to establish protocols for proposed land uses to develop designs, plans, and monitoring to promote the long-term sustainability of groundwater resources. For this purpose, long-term sustainability means that adequate groundwater quantity and quality exists to meet the long-term needs of the proposed use without negatively impacting the quality or quantity of neighboring groundwater users. Furthermore, through design and management practices, a site can provide for its long-term water needs without resort to public water line extensions, which are costly and contrary to the County's Comprehensive Plan. To achieve these purposes, the application of the standards is intended to result in better information to make decisions. These include various decisions at different scales, including: Potential purchasers of property to determine if an adequate water supply exists, Developers and landowners who intend to create new divisions to understand the constraints and opportunities of the groundwater resource, Designers of new divisions to match design and layout to groundwater characteristics, and The County to manage, monitor, and continue to build the big-picture understanding of groundwater resources to make wise land use decisions. While the information generated from the assessment standards will not guarantee the long-term productivity and/or potability of every well, the reports will represent an effort to perform a reasonable level of diligence by evaluating the sustainability of the local groundwater supply. The standards place responsibility on applicants for various County approvals to conduct some level of groundwater assessment prior to development. The County s responsibilities and roles with this program are to administer the regulatory process, maintain all relevant base maps and data bases (e.g., groundwater sensitivity areas, well data base, etc.), manage the groundwater monitoring network, and conduct basin-scale or regional groundwater studies, as funded by the Board of Supervisors, to further understand groundwater. 5

8 The groundwater assessment standards are based on a tiered approach, whereby analysis, reporting, and testing requirements are based on the nature of the proposed land use activity and whether the proposed project lies within one of the groundwater sensitivity areas outlined in the previous section. The various tier levels of groundwater assessment are outlined in Table 2. Table 2 Development Criteria with Assigned Tier Rating Criteria Not Within Area of Identified Within Area of Identified Sensitivity Sensitivity Building Permit for New Structure Tier 1: drill well prior to BP Tier 1: drill well prior to BP Creation of any development right lot (<21 acres) Creation of 4 or more lots where at least 3 lots are 5 acres or less Commercial or industrial use using less than 500 gallons/day (average) Commercial or industrial use using > than 500 gallons/day and < 2,000 gallons/day (average) Commercial or industrial use using more than 2,000 gallons/day (average) Approval of any central water supply, as defined by Chapter 16 of the County Code Tier 2: County staff review Tier 3: groundwater management plan Tier 2: County staff review Tier 3: groundwater management plan Tier 4: groundwater management plan + testing Tier 4: groundwater management plan + testing Tier 3: groundwater management plan Tier 4: groundwater management plan + testing Tier 3: groundwater management plan Tier 4: groundwater management plan + testing Tier 4: groundwater management plan + testing Tier 4: groundwater management plan + testing A detailed description of each tier level of assessment is available in the full report or can be downloaded from the County s Water Resources web page (see information at the end of the report). 5.0 Recommendations for the Groundwater Management Program The Albemarle County Hydrogeologic Assessment project was conducted to assist in protecting the quality and quantity of groundwater in the County. Work completed within Phase II of this project has resulted in the development of tools that will assist in this cause. If adopted, the groundwater assessment standards will generate additional important information that will aid in interpreting hydrogeologic conditions at the site scale. Additional assessment and monitoring data generated from site scale assessments will provide a better understanding of the groundwater resources within the County. The protection of this most important natural resource will require a continued commitment to collect, analyze and interpret hydrogeologic data to enable both interpretation of hydrogeologic conditions and to provide an enhanced understanding the hydrogeologic framework of Albemarle County. The implementation of a long-term County Groundwater Monitoring Program is recommended to further support this cause. 6

9 Establishing a groundwater monitoring program would require analysis of existing data to culminate in the preparation of a groundwater monitoring plan. The monitoring plan would identify specific monitoring goals and would be designed to aid in preparing groundwater flow direction mapping and delineation of groundwater recharge areas. Another important component of this program will be to provide continuous updating and quality control checking of the County s well data base. The sensitivity mapping completed within this assessment will require future updating to include any new public water supply sources or potential contaminant threats. The sensitivity mapping should therefore be considered a living document. In summary, the recommendations for implementing a Groundwater Management Program are as follows: 1. Adopt the groundwater ordinance and groundwater assessment standards for proposed development. 2. Provide for the long-term updating and quality control for the well data base. 3. Gradually build a groundwater monitoring program, to incorporate monitoring wells installed with new development and other monitoring locations based on specific monitoring goals. 4. Provide for regular updating of the groundwater sensitivity map and other relevant base and GIS maps. 5. Revise and update the comprehensive plan, ordinances and design standards, as new groundwater information is available. 6.0 Available Information For a copy of the full technical report on the phase 2 groundwater assessment, contact the Department of Engineering at (434) or dhirsch@albemarle.org. To download the proposed site-level groundwater assessment ordinance and design standards, go to the County s Water Resources web page (at click on Water Resources Management under the Services menu). For questions or comments, contact David Hirschman at the phone number or address listed in the first bullet. 7

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