Groundwater in Back Bay/South End Fill The groundwater in the Back Bay fill is, for the most part, contained within certain low- permeability boundaries that form the Back Bay. Addition or removal of groundwater in one location will affect wide areas within the fill.
From Gaining Ground (2003), Figure 7.9 (Courtesy of Kane Maps) 1992 Birds Eye View with 1630 Shoreline of Back Bay Study Area
Groundwater in Back Bay/South End Fill Current sources of inflow to groundwater: –Leakage from Charles River and Fens –Infiltration from rainfall –Leaky water mains –Leaky sewer pipes above El. 5 BCB –Existing recharge systems
Groundwater in Back Bay/South End Fill Current causes of low groundwater levels: –Loss of infiltration due to surface cover –Leakage into sewers below groundwater –Construction dewatering –Sump pumping from deep basements –Pumping for transportation corridors –Sump pumping from residences –Flow into lower sand layer
Groundwater in Back Bay/South End Fill But the groundwater levels are below El. 5 BCB in some zones, and nearby untreated timber piles are exposed to decay. At each zone of low groundwater levels the balance between inflow and outflow is changed due to local groundwater removal. At each location of low groundwater the causes, and therefore the methods used to solve the problem, both vary.
Purposes of Short Term Solution To raise groundwater levels in an area where they have been low for several years. To make an in-situ test of a short term recharge system. To make measurements needed to design and estimate cost of possible permanent groundwater mitigation systems. To help identify nearby causes of groundwater lowering.
View of Short Term Recharge System near Back Bay Station
Details of Short Term Solution (1 of 3) Monitor up to 30 existing observation wells including the newly installed monitoring well on Chandler Street weekly prior to starting new recharge. Install two new recharge wells, one at the north end of Cazenove St. and one on Berkeley St.
Details of Short Term Soltion (2 of 3) Recharge well on Cazenove St. at ~5 gpm using City water. Monitor effect on groundwater levels. Estimate how much the flow rate can be increased without affecting low basements. Increase recharge rate accordingly. Measure effect of increased recharge rate on groundwater levels. Follow the same steps for the well on Berkeley St.
Construction Information for Short Term Solution Construction information to be distributed to adjacent residents prior to start of construction. Construction duration will be about one week at each location. No parking in vicinity of construction during work shift hours. About one parking space at each location will be used to store equipment between work shifts. Road plates and cold patch asphalt will be placed after each work shift.
Details of Short Term Solution (3 of 3) After testing is complete for the short term solution, set the flow rates appropriately for continued recharging in the two new and two existing recharge wells. Continue groundwater level measurements and make adjustments as needed to keep the wells operational until a permanent solution is installed and operating – perhaps in three years.
Schedule Short Term Solution July to November 2006Permitting for Recharge Wells November 2006Install Recharge Wells October 2006 to December 2006 Weekly Groundwater Measurements December 2006 to December 2009 Monthly Groundwater Measurements
Herald Street Pump Station Reviewed available plans. Interviewed MBTA operations and maintenance personnel. Pump station observations made on: –August 25, 2006 and October 3, 2006. Water is not pumped automatically. Evaluating whether flow is occurring through pump station without pumping.
Suggested Solutions that may be Implemented now by Others Direct roof drains into pervious fill. Reduce pumping as much as practicable. Repair leaks in deep sewers. Repair specific leaks if practicable. Develop city-wide regulations for controlling groundwater levels.
Short Term Solution ____________________ Questions?
Steps to Select a Long Term Solution Establish Action Plan Team. Eliminate infeasible options. Obtain data needed to evaluate remaining options. For the remaining options, prepare preliminary designs and reasonable cost estimates for construction and maintenance of each option. Make detailed evaluations of selected options. Select recommended solution(s) based on evaluation of advantages and disadvantages.
Steps to Select a Long Term Solution Review MBTA plans to understand where leakage may occur. Measure water flow in manholes within track drainage system. Measure water quality in manholes and at selected pumping stations. Use results of Short Term Solution to obtain good estimate of permeability of fill and check against direct in-situ measurement in the new observation well. Use the results the Short Term Solution to help locate causes and rates of nearby groundwater removal.
Possible Long Term Solutions for the MBTA Reduce pumping as much as practicable. Recharge pumped groundwater into fill. Build buried walls or do local grouting. Recharge roof drains directly into fill. Baffles in manholes. Line drain pipes. Combinations of above.
Steps to Select a Long Term Solution Evaluate practicable solutions to mitigate or eliminate effect of groundwater pumping. Select a solution that is effective, environmentally appropriate, and has acceptable initial and long- term costs.
Draft Schedule Long Term Solution Recommendation July 2006 – December 2007 Data Acquisition and Public Involvement January 2007 – February 2007 Data Evaluation and Analysis, and Public Involvement March 2007 Recommend Long Term Solution and Public Involvement
Draft Schedule Long Term Solution Fall 2006 – Winter 2006 Evaluate, Recommend, Public Involvement Winter 2006 – Spring 2007Plan, Design, Public Involvement Spring 2007 – Winter 2007Bid, Permit, Public Involvement Winter 2007 – Winter 2008Construct, Public Involvement Winter 2008 – Spring 2009 Observe, Monitor, Public Involvement Spring 2009 – Winter 2009 Phase Out Short Term Solution, Public Involvement