Ground-Water Studies for the Real World

Four Steps for Basic Ground-Water Studies

1. What are the Rocks? (Geologic formations)

2. How Far Down Is the Water? (Water Level Data)

3. How Fast Does the Water Move? (Hydraulic Conductivity and Transmissivity)

4.  Which direction is the ground water moving?  (Water table maps and potentiometric contours)

5. How Much Water Do I Have and How Long Will the Water Last? (Well Yield and Safe Yield)

 

1. What are the Rocks? (Geologic formations)

Geology controls the movement of water underground. The geology of many areas has been mapped, but unfortunately the geology of many areas has not been mapped. Where the geology has been mapped (the smaller the scale, the better--such as 1:24,000), hydrogeologic studies are much easier to accomplish. Any scale geologic map is better than none.

2. How Far Down Is the Water? (Water Table Data)

The depth to water underground is very important for hydrogeologic studies. The depth to water can be affected by the height of hills, mountains, and valleys; the depth of valleys and canyons; and the distance to streams, lakes, and irrigation ditches. Water table data are collected from as many wells as possible, and a water table map is drawn that shows contours of the water table. Itís like an artistís rendition of the shape of the water table. Since water flows perpendicular to the water table contours, the direction that the water moves is described by the water table map.

3. How Fast Does the Water Move? (Hydraulic Conductivity and Transmissivity)

Ground-water movement occurs primarily through primary pores in the aquifer and fractures in bedrock.  The speed that water moves underground depends on the tightness of the rocks and soils.  The tightness of the rocks depends on Step 1--Geologic Information.  The speed of the water movement also depends on Step 2--Water Table Data. Acquiring information on how fast the water moves can be difficult, but there are many approaches that can be taken.  Some approaches require lengthy pumping tests, others only require short injection tests.  Resulting information can provide hydraulic conductivity and transmissivity data. Any data are better than none.

4.  Which direction is the ground water moving?  (Water table maps and potentiometric contours)

Ground-water flows from higher head to lower head, and the head of the aquifer can be represented by the shape of the water table or the potentiometric surface of the aquifer.  On bedrock terrains, the primary pores of the aquifer may provide water to the fractures; however, the primary avenues for movement of water to wells, streams, and springs occurs through fractures. In a fracture-dominated system, ground-water flow is less influenced by the dip of the bedrock. Ground water flows from high potentiometric head to low potentiometric head, and the water levels in wells are indications of the head in the fractured-rock aquifer.  Locations of springs and surficial expressions of streams also can be indicators of head--streams can be recharge points (or lines) for an aquifer, and springs are discharge points from the aquifer.  The gradient or slope of the potentiometric surface is the driving force for ground-water flow. 

5. How Much Water Do I Have and How Long Will the Water Last? (Well Yield and Safe Yield)

The yield of a well is simple--How much water can be pumped from a well without drying out the well?  Many people believe that a well must be pumped continuously for 12 or 24 hours to determine the yield of a well.  Household water use may only total 2 or 3 hours a day, and the use is spread throughout a 12 to 16 hour period.  This pumping must be done before household pumps and pressure tanks are installed, because the pressure switches on storage tanks turn the pump on and off during pumping, and the well cannot be pumped enough to provide adequate yield information.

Safe yield needs to be determined for wells that supply water for numerous houses.  Safe yield is determined for the maximum amount of water needed during the driest year.  Safe yield depends on the Four Steps, plus calculations and computer programs that determine the effects of rainfall, snowmelt, evaporation, long-term water storage, and climate.

What Is Needed For Real World Hydrogeologic Studies?

--Our goal is to collect scientific data according to protocols established by Local, State, and Federal agencies.

--Identify the Problem Areas. Focus on smaller areas, and collect detailed data according to the Four Steps.

--Collect as much data as possible; long-term monitoring is essential to identify trends.

--Evaluate and interpret data using state-of-the-art methods.