Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 57

2.5.7 Non-Clastic Chemical Depositional Environments
As noted earlier, chemical sedimentary deposits form as minerals precipitated from water
through organic or inorganic processes. Identification of sedimentary deposits from chem-
ical processes requires the identification of the primary mineral.
Common evaporite deposits include halite and gypsum. All water bodies on the surface
and in aquifers contain dissolved constituents including salts. For an evaporite deposit to
form, the water must evaporate into the atmosphere, causing the minerals dissolved in the
water to precipitate. Evaporites generally form when water accumulates in closed basins
in arid and semiarid environments. Enclosed basins in arid environments are favored
because they provide high evaporation rates due to low humidity and lack an outlet for the
water to drain, which essentially traps the water. Examples of extensive evaporite deposits
include areas in and around the Great Salt Lake of Utah and parts of Nevada.
Many limestone deposits form from the accumulation of shells or precipitants from
microorganisms. The deposition of limestone strata is thus a byproduct and indicator of
biological activity in the geological record (Blatt and Tracey 1996; Taylor and Wilson 2003).
Over time, these deposits build and sometimes become hundreds of feet thick.
2.6 Disturbance
Disturbance of sedimentary deposits occurs constantly from the onset of sediment forma-
tion. There are several anthropogenic and natural types of disturbance, and when con-
ducting studies in an urban environment, these should be described and evaluated.
2.6.1 Anthropogenic Disturbance
One common concern for any investigator evaluating shallow unconsolidated deposits is
whether any disturbance has occurred since deposition. This concern is ratcheted up in
urban and developed areas since anthropogenic disturbance often includes excavation
or filling.
Disturbance can also occur naturally, although at times it may be difficult to determine
whether the disturbance is natural or anthropogenic. For instance, spoils from dredging his-
torically were spread over the land surface in low areas to enable the development of these
areas. Dredging a river bottom and spreading the material over the land surface is accom-
plished by pumping a combination of river water and bottom sediment to the desired area
of disposal. Using water as the transport agent for the sediment may create some difficulty
in identifying the source of the disturbance because the water and this mixture of sediments
and other materials can leave depositional structures similar to natural deposits. Therefore,
special care should be undertaken when evaluating sediment layers in urban settings.
Often, this type of activity can be readily observed—but this is not always the case.
Typically, to evaluate the existence of anthropogenic disturbance in a specific area, geolo-
gists rely on a few clues for assistance. Some of these clues include
• Evidence of historical development
• Evidence of landfilling
• Evidence of grading or land surface disturbance