By looking at soil and rock types, thickness of soil and rock layers, and depth to groundwater, hydrogeologists can determine a lot about the groundwater beneath.

I would dare say that less than half of the practicing hydrogeologists know the difference between hydrogeology and geohydrology. There are many other less than well-known definitions in the field. Let's go over a few so we can show off our knowledge the next chance we get.

Hydrogeology vs. Geohydrology

To make it easy, the second half of the word is the science used to understand the first half of the word, i.e., a hydrogeologist uses the physical geological characteristics of the strata or rock to infer the hydrologic characteristics of the material. Clays and silty clays are usually hydrologic confining units which "restrict" vertical movement of ground water. On the other hand, geohydrology (less commonly used) is the interpretation of the geology from the hydrologic characteristics of the strata or rock, i.e., a clean sand or permeable limestone is likely to be an aquifer.

Another Pair

Two words commonly and inappropriately interchanged are piezometric and potentiometric. Piezometric simply is the unconfined water table or the first saturated zone encountered with depth from land surface. Potentiometric is a hydrologic condition where the subject aquifer is overlain by a confining clay (clay, dolomite, shale, etc.) and the water rises, or would rise above, the top of the aquifer if a well casing were sealed tightly into the top of the aquifer (even if it is only one inch). Remember, if the water level has the potential to rise above the aquifer, it is said to have a potentiometric surface. An aquifer in certain areas may change from confined to unconfined during a single year depending upon factors affecting water levels like rainfall, pumping, etc. Potentiometric also is associated with another word - artesian - which is the ability of the water level to rise above land surface, such as in the case of a spring or flowing well.

The actual depth of the water level has nothing to do with the word. In west Texas, the first saturated zone encountered may be 500 feet below land surface. However, along the Gulf Coastal Plain of the United States, confined aquifers may be only a few feet below a competent clay layer along the banks of rivers located in karst terrains.

Cluster vs. Nester Wells

These two words often are interchanged incorrectly in their usage. A nested monitoring well is a single well "open" to multiple depths. A well cluster (there is no such thing as a "cluster well") is a group of wells very close together with each well open to a single zone. A well cluster may consist of a S (shallow), I (intermediate) and D (deep) well at one location.

Next time we will discuss groundwater vs. ground water. Not really, because I haven't figured that one out yet.