Mapping Aquifer Stress, Groundwater Recharge, Groundwater Use, And
Total Page：16
File Type：pdf, Size：1020Kb
WATER SCIENCE SERIES Mapping Aquifer Stress, Groundwater Recharge, Groundwater Use, and the Contribution of Groundwater to Environmental Flows for Unconfined Aquifers across British Columbia Tara Forstner, Tom Gleeson, Leigh Borrett, Diana M. Allen, Mike Wei and Andarge Baye August 2018 No. 2018 -04 The Water Science Series are scientific technical reports relating to the understanding and management of B.C.’s water resources. The series communicates scientific knowledge gained through water science programs across B.C. government, as well as scientific partners working in collaboration with provincial staff. For additional information visit: http://www2.gov.bc.ca/gov/content/environment/air-land-water/water/water-science- data/water-science-series. ISBN: 978-0-7726-7233-9 Citation: Forstner, T., Gleeson, T., Borrett, L., Allen, D.M., Wei, M., and Baye, A. 2018. Mapping aquifer stress, groundwater recharge, groundwater use, and the contribution of groundwater to environmental flows for unconfined aquifers across British Columbia. Water Science Series, WSS2018-04. Prov. B.C., Victoria B.C. Author’s Affiliation: Tara Forstner, B. Sc. Tom Gleeson, PhD. Leigh Borrett, B.Eng. University of Victoria Diana Allen, PhD., P.Geo. Simon Fraser University Mike Wei, M.Sc. Andarge Baye, M.Sc B.C. Ministry of Environment and Climate Change Strategy © Copyright 2018 Cover Photographs: Figure from report – aquifer stress for the Lower Mainland and Vancouver Island. Acknowledgements This research is supported by the B.C. Ministry of Environment and Climate Change Strategy. This report benefited from reviews by Klaus Rathfelder and Christine Bieber. A special thank you for the personal communication, community of practice discussions, and feedback: Jaro Szczot, Mike Simpson, Ron Ptolemy, Jennifer Turner, Antonio & Sylvia Barroso, and Jun Yin. Disclaimer: The use of any trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented for discussion purposes only. Funding assistance does not imply endorsement of any statements or information contained herein by the Government of British Columbia. WATER SCIENCE SERIES No. 2018 - 04 i EXECUTIVE SUMMARY Key Finding: One in every five unconfined aquifers in the province is likely stressed. Regions of priority for groundwater management include the Okanagan, the Lower Mainland and Vancouver Island, as well as isolated aquifers in other regions. Need for this Study: Groundwater is a critical source of freshwater supporting residential, commercial, industrial and agricultural sectors within British Columbia (B.C.). The Province has mapped and classified more than 1100 aquifers across B.C., but the level of development for each aquifer has always been subjectively based on well density or the mapper’s knowledge of groundwater use. This project focuses on the synthesis, spatial analysis and modelling of existing data to map stress for unconfined aquifers across the province and develop an aquifer-scale decision support tool to assist water managers in decision making, prioritization of high risk aquifers for detailed studies, promote awareness of groundwater resources across the province, and encourage the protection and management of this valuable freshwater resource. The scope of this project is to derive aquifer-scale estimates of annual volumes for groundwater withdrawal, recharge, and groundwater’s contribution to environmental flows as a means to provide screening level estimates of aquifer-scale stress using the groundwater footprint. Groundwater Use: This report estimates groundwater use across B.C. for all the major groundwater use sectors and maps this groundwater use for each aquifer in the province for the first time. Data on major sectors of use was synthesized from provincial and national sources and spatially downscaled and interpolated to derive groundwater use volumes for currently mapped aquifers. Groundwater use was first classified based on means of distribution either through a municipal water distribution systems or self-supplied through private wells, and secondly, by major groundwater use sectors namely, domestic, commercial, industrial, irrigated agriculture, and finfish aquaculture. The methodologies used in deriving the spatially distributed groundwater use volumes are different for each sector based on the data availability and scale of reporting. Results suggest that B.C. uses a total of ~562 million cubic metres of groundwater annually. The largest annual groundwater use by major sectors is agriculture (38%), finfish aquaculture (21%), industrial (16%), municipal water distribution systems (15%), and domestic private well users (11%). This study is a preliminary assessment, as the majority of the groundwater volumes were unreported per sector, and therefore, different methodologies are used to interpolate available data. Groundwater Recharge: Recharge is estimated using two approaches; a generalized aquifer-scale method, using the Hydrologic Evaluation of Landfill Performance (HELP) and modelled recharge outputs from PCR-GLOBWB, a global hydrologic model. Results show that generally recharge predictably varies with precipitation and that the average recharge is 462 mm (32% of precipitation) for the aquifer-scale HELP method and 393 mm (33%) for the global hydrologic model. The generalized methodology results in annual recharge rates that are consistent with the more localized studies of the Abbotsford-Sumas aquifer in humid southwestern B.C. and the Grand Forks aquifer in semi-arid south-central B.C. Although the results are consistent with previous localized recharge modelling, as well as expected patterns of recharge across the province, these recharge estimates should only be used for regional to provincial scale groundwater resource management rather than localized analysis. WATER SCIENCE SERIES No. 2018 - 04 ii Groundwater’s Contribution to Environmental Flows This study estimates groundwater’s contribution to environmental flows across the province for this first time using two separate approaches. The first approach uses the groundwater presumptive standard, which is a general standard for managing groundwater pumping. The second method introduces a novel approach for estimating the contribution of groundwater to environmental flows using the existing B.C. environmental flow needs framework and an understanding of low flow zone hydrology. In general, both methods show larger contributions from groundwater to environmental flows in the Lower Mainland and southern Vancouver Island compared to the Interior. As both methods rely on modelled baseflow and streamflow, respectively, uncertainties increase in mountainous terrain where the models may not fully represent local aquifer dynamics. Aquifer Stress: For each aquifer, the groundwater footprint (expressed as the unitless ratio of groundwater footprint to aquifer area or GF/A) is calculated four times; using results from each of the two methods used to estimate recharge and each of the two methods used to estimate the groundwater contribution to environmental flows. None of the methods can be categorically considered more scientifically robust, so all four of the calculated groundwater footprints are reported and combined into three mapped categories that also highlight the uncertainty of the results: (a) more stressed (highly certain) if all results suggest aquifer stress; (b) more stressed (less certain) if some results suggest aquifer stress; (c) less stressed if none of the results suggest aquifer stress. The groundwater footprint, which is an indicator of groundwater stress with a single cut-off: GF/A > 1 suggests an aquifer is more stressed, and GF/A < 1 suggests less stressed. There is no scientific basis to interpret the calculated values more finely. Given the significant limitations and uncertainties in the input parameters and single cut-off, the calculated values of the groundwater footprint should not be over-interpreted. For example, aquifers with GF/A of 2 or 10 should both be considered 'more stressed' and provoke similar management decisions. Of the unconfined aquifers (n = 404) in the province, 43 aquifers (11%) are stressed with high certainty, 32 aquifers (8%) are stressed with low certainty, 296 aquifers (70%) are less stressed, and 29 aquifers (11%) were not included due to missing parameters or issues where modelled recharge was less than environmental flows. Recommendations: We provide a number of recommendations that could significantly improve the conjunctive management of surface water and groundwater in the province including: establishing new systems for tracking groundwater use and linking this with aquifers; improving the observational well network to better quantify recharge; develop a new environmental flow regulation, incorporating many of the elements of the existing EFN policy, but more holistically incorporating the importance of groundwater; testing the new methods for quantifying the groundwater contribution to environmental flows to determine if they could be used in operational water management; and using the aquifer-scale decision support tool to assist water managers in decision making, prioritizing high risk aquifers for detailed studies, promoting awareness of groundwater resources, and encouraging the