Streamflow and Groundwater

Upper Watershed Groundwater Management

Project completed September 2003

Background

The Charles River watershed is the most densely populated watershed in New England.  While the lower half of the watershed has been developed for many years, the upper watershed has recently experienced rapid population growth, leading to stressed water resources.  The upper watershed towns are facing increasing demands on their water supplies while there is increasing concern about the depletion of aquifers and low flows in the river and its tributaries.  The need for long-term water management planning has become increasingly important.

The primary source of water supply in the upper watershed is groundwater aquifers. Several towns share aquifers potentially leading to conflicts over water rights.  In addition, water withdrawals can lead to noticeable impacts on rivers and ponds, because the highest demand period occurs in the summer when stream flow and water levels are low.

Increased development can impact water resources in the following ways:

·        Increased withdrawals to meet the increasing population demand can directly lower base flow (non-storm flow) or aquifer storage.

·        More impervious surfaces (rooftop, roads, and parking lots) which results in increased storm runoff (not dealt with in these studies) and decreased base flow and aquifer storage via diminuished recharge.

·        Expansion of sewer distribution systems that can export water far from its source to a wastewater treatment facility.

1998-2002 Study

CRWA teamed with the MA state agencies (DEP, DCR, EOEA) and the federal government (USGS) to procure funding for a project to quantify the effects of withdrawals on streamflow and develop alternatives for managing water resources.  The project approach was to calibrate a MODFLOW groundwater model to measured field data then use the computer-based model to evaluate management strategies.

USGS installed a network of monitoring wells to determine the elevation of the water table.  USGS and CRWA monitored the groundwater elevations and measured streamflow monthly for one year to provide the calibration data set.  In addition, information on municipal and commercial water withdrawals and wastewater discharges were compiled for the study period of 1989–1998.  CRWA played a crucial role in developing the recharge grid used by the model.

The MODFLOW model was used to simulate ground water movement in the sand or gravel aquifer areas as well as interactions between the clay or till contributing areas and associated surface waters.  The model predicts base flow in response to seasonal recharge, conductivity of the aquifer, and point withdrawals or discharges.  The model was used to simulate the effects of existing or planned water withdrawals, wastewater discharges, and aquifer recharge strategies on base flow and water levels in the upper watershed.  Results from the model will also be used by DEP personnel to assess the impact of future water withdrawal permits.

Management scenarios were developed with oversight from a technical advisory committee.  In this study, the alternatives evaluated were:

·        increased water withdrawals to current permit levels including proposed wells:
o       water withdrawals increased from 10.1 to 15.6 mgd
o       base flow reductions ranged from less than 5% to more than 60%
o       base flow reductions were larger in tributaries than the Charles River

·        increased recharge in the Mine Brook tributary:
o       use of treated wastewater increased base flow by 12%
o       use of treated wastewater offset the impact of increased withdrawals (above)
o       use of residential rooftop runoff augmented September base flow by 3%
o       recharge impacts were lower than expected because Mine Brook has few people and households

Municipalities played a vital role in the project and are responsible for implementing water resource management plans.  CRWA coordinated five stakeholder meetings with interested municipal representatives and the project ream.  The meetings addressed project components, interim results, and provided feedback.  On March 4, 2003, the USGS publicly released the Upper Charles Ground-Water Model to the towns in the upper watershed at a breakfast gathering at Tri-County Regional Vocational School in Franklin.

This study culminated with the production of a USGS Water-Resources Investigations Report (Desimone, 2002).

2002-2003 Study

A follow-up study was conducted by USGS to further evaluate water management scenarios through the use of optimization techniques.  Optimization strategies involved either maximizing streamflow while holding withdrawals constant or maximizing withdrawals while holding streamflow constant.  The study also evaluated a non-pumping scenario which can be used as a surrogate for the natural condition.

Estimates of natural base flow varied widely with Hopping Brook being the lowest (August flow 0.14 cfsm) and Mill River the highest (August flow 0.90 cfsm).  These differences are mostly related to the recharge potential of the surficial geology and the storm flow contribution to base flow from wetlands.  The most promising water-management strategies to increase streamflow or water supply included wastewater recharge to the aquifer; altered management of pumping well schedules; regional water-supply sharing; and water conservation.  In CRWA’s opinion, other alternatives like stormwater recharge and reduction of sewer infiltration and inflow were not adequately addressed in this study.

This study culminated with the production of a USGS Water-Resources Investigations Report (Eggleston, 2003).

Conclusions

Despite the pressure of increasing development in the upper Charles River watershed, there are water management alternatives that will help alleviate streamflow impacts and work towards restoring the natural streamflow.

Water sharing, where towns use water jointly instead of being self-supplied, and altered management of pumping well schedules are low cost water management alternatives.  Water sharing results in preferred withdrawals from water-rich tributaries especially during the summer.  Revised pumping well schedules make use of the lag time between well and stream.  In the summer, more is pumped from the upland wells and less from the streamside wells while the reverse strategy is employed in the winter.

Water conservation is a medium-cost alternative that has a direct benefit on restoring base flows because it can reduce the large point impact of a well on a nearby stream.  This alternative is emphasized in DEP’s 2004 Water Policy and current water withdrawal permits.

These water management alternatives are all short-term in the face of continued population increases.  Real restoration on the natural water cycle can only be achieved by offsetting the impacts of development on base flow by recharging properly treated stormwater and wastewater into aquifers.  Although some of these alternatives are costly, they are the only long-term solutions, and will have to be implemented in the future as water conflicts and streamflow impacts increase.

References

DeSimone, L.A., Walter, D.A., Eggleston, J.R. and Nimroski, M.T., 2002.  Simulation of Ground-Water Flow and Evaluation of Water-Management Alternatives in the Upper Charles River Basin, Eastern Massachusetts.  USGS Water-Resources Investigations Report 02-4234 (http://pubs.usgs.gov/wri/wri024234).

Eggleston, J.R., 2003.  Evaluation of Strategies for Balancing Water Use and Streamflow Reductions in the Upper Charles River Basin, Eastern Massachusetts.  USGS Water-Resources Investigations Report 03-4330 (http://pubs.usgs.gov/wri/wri034330).