Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 44

TABLE 2.2
Grain Size Classification

Wentworth Size Class Millimeters Inches

Boulder 256 or greater 10.08 or greater

Cobble 64-256 2.52-10.08

Pebble 4-64 0.16-2.52

Very coarse gravel 32-64 1.26-2.52

Coarse gravel 16-32 0.63-1.26

Medium gravel 8-16 0.31-0.63

Fine gravel 4-8 0.16-0.31

Granule (very fine gravel) 2-3 0.08-0.16

Very coarse sand 1-2 0.04-0.08

Coarse sand 0.5-1 0.02-0.04

Medium sand 0.25-0.5 0.01-0.02

Fine sand 0.125-0.25 0.005-0.01

Very fine sand 0.0625-0.125 0.002-0.005

Silt 0.004-0.0625 0.0002-0.002

Coarse silt 0.031-0.0625 0.001-0.005

Medium silt 0.0156-0.031 0.0005-0.001

Fine silt 0.0078-0.0156 0.0003-0.0005

Very fine silt 0.0039-0.0078 0.0002-0.0003

Clay Less than 00039 Less than 0.0002

Source: New Jersey Department of Environmental Protection

(NJDEP). 2005. Field Sampling Procedures Manual. Trenton,
NJ: NJDEP.

1mm

FIGURE 2.21
Example of a sub- to well-rounded, well-sorted, medium-grained, quartz sand. The light colored mineral grains
are quartz. (Photo by Daniel T. Rogers.)

rainwater dissolves small quantities of carbon dioxide, so when the rain reaches the
ground, it is a weak carbonic acid. As water migrates through the soils and sediments,
the strength of the acid may increase significantly through the addition of carbon dioxide
created by the bacterial decay of vegetation. Once carbonic acid forms, it ionizes to form
hydrogen and bicarbonate ions. Figure 2.22 shows the chemical reactions leading to the
formation of carbonic acid, hydrogen, and bicarbonate ions.