At the January 2011 Planning Board meeting the groundwater report was discussed. In response to questions raised at that meeting Gary Witt from Wright Water Engineers give a Groundwater 101 presentation at the May 5th Planning Board meeting ( agenda). The presentation begins 22 minutes into the audio.
In Fall 2011 the developer submitted a letter detailing their response to the questions that have been raised.
January 5th 2012 the neighborhood group gave a Groundwater 102 presentation to the Planning Board.
Our OverviewThe conlusions of the May 2010 groundwater report are given: (emphasis added)
Because all other summer mechanisms are unchanged, the only logical conclusion is that development in the Project area will lead to a reduction in basement sump pumping within the adjacent neighborhoods. There are simply no mechanisms associated with the development that could cause the summer pumping rates to increase.
The most important conclusion of this study is that the proposed housing development on the Hogan-Pancost Property can only lead to a decrease in sump pumping in nearby residences. This conclusion is based on accepted hydrologic principals and sound logic. The ground water flow model provides a quantitative estimate of the decrease in the sump pumping rate. Thus, two methods of analyses indicate the same results; that project development cannot logically lead to increased basement sump pumping.The Fall 2011 response letter report states (emphasis added):
Question: What can cause ground water levels to rise and fall?
Response: Ground water levels rise and fall in response to the amount of recharge that becomes ground water.
Question: What effect will the development of the Hogan-Pancost property have on the local ground water level?We feel that this solitary focus on recharge rates ignores very real and well documented mechanisms in which development can and will affect groundwater levels.
Response: It is not possible for the development on Hogan-Pancost property to significantly influence the local ground water level because the lateral extent of the underlying ground water is so extensive and the property represents only a small fraction (0.03%) of the total watershed area. The property owners can only control on-site recharge. Recharge to ground water from the development area will attempt to mimic the current recharge condition. As a result, development will not increase recharge to ground water or increase the elevation of the local ground water level. Piping Dry Creek Ditch No. 2 will reduce the local recharge to ground water.
For example, the "bible" of groundwater engineering, Construction dewatering and groundwater control: new methods and applications By J. Patrick Powers, Christine J. Herridge, Arthur B. Corwin, Paul C. Schmall describes in detail ways in which groundwater can be affected (both raised and lowered) by construction.
11.11 PERMANENT EFFECTS OF STRUCTURES ON THE GROUNDWATER BODYThis book also has an extensive section on groundwater modeling techniques. They offer the following advice which is particulary relevant to the Hogan/Pancost groundwater report.
Major projects, such as sewers or mass transit systems, can create significant changes in groundwater levels and movements under a city. Algnments perpendicular to the general direction of groundwater flow can create underground dams, resulting in higher levels upstream and lower levels downstream. Relieved sections of retained earth structures can cause permanent depressions in the groundwater table, as can structures that are imperfectly waterproofed and must be pumped on a continuous basis. Sewers running parallel to the direction of groundwater flow, if they have been laid in gravel bedding, may cause permanent lowering of the groundwater table since they act as draings. Where deep building foundations are designed with relieved slabs and walls, the pumping of the relief system will usually depress the water table permanently.
... Long-term changes may or may not be detrimental.
The first step in the modeling process is to define the nature of the problem. Questions such as the following must be posed:In the paper The impacts of urbanization on groundwater systems and recharge Professor John Sharp, an expert in the field of urban impacts on groundwater, states:
These limitations must be understood before embarking on a modeling venture, and be retained in the analysis and evaluation of the model results:
- Is the aquifer contained or water table?
- Do we need a steady-state or a transient solution?
- Will the pumping devices fully penetrate the aquifer?
- What are the assumed values of transmissivity T, hydraulic conductivity K, thickness B, storage coefficient C, and anisotropy . Do we expect them to be relatively uniform throughout the flow regime?
- The relialability of the above assumptions must be assessed. Are they based on pumping tests, local experience, water supply records, or only test borings and sieve analyis?
- Does the aquifer receive recharge from a surface water body? Is it recharged by leakage from aquifers above, below, or to one side, or by surface infiltration?
- A model is only an approximation of a real groundwater system
- High-powered mathematics and complex graphics do not make up for poor data or a poor understanding of the dynamics of groundwater flow.
The network (or reticulation) of water mains, sewer lines, electrical and telephone conduits, storm drains/sewers, subways, and other sub- surface systems is one of the major alterations to the hydrogeology of an urban area.He goes on to describe the changes in permeability between a backfilled utility trench and the surrounding soils:
They have found 2-4 orders of magnitude increase in permeability in the trench backfill. The paper states:
This highly altered permeability field can lead to the following:Sharp even casts doubt on the long held notion that urban development can lead to reduced recharge rates.
- Altered groundwater flow systems.
- Maintenance of stream base flows and spring flows during times of limited rainfall or, alternatively.
- Reduced increased spring flows, if flow is diverted from spring orifices.
- Diversion of groundwater to different streams or catchments.
- Artificial recharge caused by leakage of water, sewage, and storm waters along the utility lines.
- Difficulty in predicting, modeling,and remediating subsurface contamination.
- Creation of multiple contaminant plumes that can migrate in different directions than might be predicted from standard analyses.
- Utility trenches and mains/sewers serving as "French drains" to limit rising water tables.
Although it is commonly stated, that groundwater recharge is reduced with urbanization because of the increase in impervious cover, the reverse is the more common condition - urbanization increases ground water recharge.In EFFECTS OF URBANIZATION ON GROUNDWATER SYSTEMS Krothe summarizes the impacts seen:
Of course, it should be noted that the recharge and changes to recharge in a city also vary spatially. It may be decreased in one portion of an urban area because of increases in impervious cover and soil compaction and increased in other areas because of a number of other factors. These include leakage from water mains, sewer lines, and storm drain systems; the effects of storm water detention ponds and artificial recharge; irrigation return flow from lawns, gardens, and parks; losing streams; and the fact that impervious cover is not all impervious.
Urbanization has dramatic effects on groundwater systems with ramifications for water management.In the paper Rise of the groundwater table when flow is obstructed by shallow tunnels Marinos and Kavvadas describe how the the groundwater table can be affected when flow is obstructed by shallow tunnels
Increases in permeability within the utility systems are documented up to 10 orders of magnitude greater than the natural materials, although 2 to 4 orders are more common.
Prediction of groundwater flow and contaminant transport is difficult; and remediation in such systems can be problematic.
For typical values of the hydraulic gradient (0.5-5%), the predicted water table rise is in the order of 1-10% of the tunnel height for tunnels located just below the original level of the water table. For tunnels located at some depth below the water table or for partially submerged tunnels, the magnitude of the water table rise is lower.In Urban groundwater-- meeting the challenge Howard describes:
Thc covering and replacement of natural rocks, soils, and vegetation by pavements, foundations, buildings, metallic structures, dams, tunnels, and other structures has a profound impact on the hydrology of an area. The urban underground is an intricate and rapidly changing network of tumels, buried utilities, garages, and other buried structures that disturb the natural structure of the ground and alter its porosity and hydraulic conductivity.Other overviews include Humans as geologic agents , Effects of urbanization on groundwater systems and Issues In Urban Hydrogeology .
Advanced simulation and modelling for urban groundwater management the authors
describe the impacts that foundation walls can have on groundwater flow:
Foundations and cutoff walls: In aquifers, foundations can significantly affect groundwater flow patterns, producing both damming and funneling features.
The impacts of soil and fill is described in
in Groundwater and the Environment: Applications for the Global Community
Another example of causing a rise in groundwater in groundwater level at a regional scale can be from fields of piled earth, made during the total building up of microrayons.
In the pile field zone, the ground is so compressed that its filtration properties decrease almost by an order. Thus, considerable backing-up due to the damming effect of a pile field occurs and a vast area of flooding upstream