Overburden thickness and bedrock surface topography . The type and 

 thickness of the unconsolidated material that overlies the bedrock and the 

 corresponding yield of groundwater are depicted in figure 2-26. Thick, 

 permeable, and saturated overburden supplies groundwater to shallow wells and 

 recharges water to underlying bedrock aquifers. Figure 2-26 can be used in 

 conjunction with surficial geological maps to estimate the relationship 

 between the thickness and type of surface materials and water yield. Caswell 

 (1977) has found a strong correlation between types of overburden material and 

 bedrock well yield. 



Because most water purification processes take place in the unconsolidated 

 sediments overlying the bedrock, local bedrock aquifers in thinly overburdened 

 areas are more susceptible to pollution from surface sources of contamination. 

 For this reason, waste disposal areas are least acceptable on thinly covered 

 bedrock. 



Information about the distribution of bedrock surface elevation is used to 

 determine probable groundwater flow in the overburden just above bedrock. 

 Groundwater in overburden generally flows at right angles to the bedrock 

 surface contours. 



Potentiometric surface of bedrock wells . The elevation to which 

 groundwater will rise in an artesian well drilled into the bedrock aquifer is 

 defined as the potentiometric surface. A map of this surface indicates the 

 potential for groundwater flow in the bedrock from any point to any other 

 point. The physical properties of bedrock vary from area to area and 

 groundwater flow has been shown to depend more upon bedrock fracture patterns 

 than upon bedrock characteristics. Potentiometric surface maps, available 

 from the Maine Geological Survey, represent ideal flow conditions showing that 

 high flow potentials occur at topographic highs and that low flow potentials 

 generally occur in topographic valleys (figure 2-27). Valleys tend to be 

 areas of groundwater discharge, while high elevations and slopes are recharge 

 areas. Groundwater divides generally coincide with topographic divides. 



Potentiometric surface maps could be used to predict flow characteristics of 

 groundwater systems, taking into account the fact that fracture systems will 

 affect groundwater flow differently than the flow potential surfaces indicate. 

 A knowledge of the local fracture system is therefore necessary to determine 

 general groundwater conditions. 



Ground and surface water contamination . Contamination of ground and 

 surface water supplies occurs through natural flux of particulates and 

 dissolved ion species, direct introduction of wastes into water bodies, and 

 introduction of contaminants through surface soil runoff. 



Natural contamination of groundwater supplies has been documented by Caswell 

 (1977), who lists the following occurrences of contamination in the Maine 

 coastal zone: 



1. Salt water intrusion in fracture-controlled aquifers occurring near 

 the ocean (figure 2-28). 



2-51 



10-80 



