A Groundwater, Water Table Inspection is designed to prevent basement flooding, septic systems leaching field failure or other issues caused by a high water table. Water that soaks into the ground is pushed by gravity and percolates until it reaches an impervious layer it cannot penetrate, then it begins to move in a lateral direction. This underground flow is referred to as groundwater, and the top surface of groundwater is called a water table. The level of the water table will vary depending on the amount of rainfall, so it might be several feet higher after a prolonged rainy period than during a prolonged spell of dry weather.
A high water table, however, can result in a flooded basement or failure of the septic systems leaching field. In many areas across the United States the seasonal high water table is just two to five feet below the surface. In such areas, houses should be built on a slab or over a crawl space, not over a full basement, otherwise water will penetrate into the basement. Depending on the soil, even when the water table is several feet below the basement floor, some water might still seep into the area as a result of the capillary rise of groundwater.
Moreover, homes located in these areas where the seasonal high water table is just a few feet below the ground surface should not have septic systems for waste disposal, but be connected to a sewer system. For a septic system to operate properly, the water table during the wet season should be at least four feet below the bottom of the leaching field or seepage pit.
Groundwater, Water Table Inspection
The top surface of flowing streams, rivers, lakes and oceans consist the water table, so a terrain that gradually slopes into the waterway will have a high water table. Homes located in these areas are not just vulnerable to problems associated with a high water table, but are also prone to flooding. The water table tends to follow the general contour of the land and in some areas it may intersect the ground surface, forming marshy wetlands which are costly to drain and provide a breeding space for insects.
Excessive grading or reshaping of the ground surface can change the natural drainage patterns and cause groundwater to seep to the surface. Also, in areas with a seasonal high water table, if the land slopes toward one side of the house, subsurface water will flow toward the house, in addition to surface-water runoff. If allowed to accumulate around the foundation, this water can seep into the basement or crawl space. Generally, this condition can be controlled by installing a curtain drain in the hillside parallel to the house, in order to divert the water away from the house.
A curtain drain is made of a perforated drainpipe installed in a trench that is filled with gravel and covered with soil. This trench extends several feet beyond the house, with one end leading to a proper disposal area. Though the popular belief is that the perforations in the pipe should be facing upward, they should actually be facing downward. As the subsurface water level rises, it enters the holes along the length of the pipe. Water always takes the path of the least resistance, so once inside the pipe it flows into the outlet, which must be located away from the house and must be obstructed. The outlet should have an animal screen to prevent a small animal from entering and becoming lodged.
In areas with a seasonal high water table or a risk of surface water accumulating around the foundation, it is best to have foundation footing drains (perforated drain-pipes) installed parallel and adjacent to the foundation footing. Footing drains are designed to channel the water that accumulates around the foundation to another location. Footing and curtain drains must have a free-flowing outlet or discharge into a sump pit where the accumulated water can be pumped to the desired location.