17 
b. There is a bottom mixed layer, defined by the first or second density slope 
observation from the bottom sediment-water interface being less than 0.2 
kg/m 3 /m 
c. The next density slope observation is positive. 
U.S. EPA (2004a, see pg. 87) also provided the procedure for calculation of the 
vertical density profile. 
These two decision rules remain unchanged. The detailed step-by-step procedure for 
applying the two decision rules has been provided here. 
Determining the vertical density gradient and defining pycnocline depths requires a 
vertical profile of salinity and water temperature measurements collected at multiple 
depths and computed as follows: 
1. Sort the vertical profile of data from the water surface downwards through the 
water column. 
2. For each depth at which there are measurements, calculate a water density 
value as crT, or “sigma T”, using water temperature and salinity measurements 
for that depth. Use the following method and equations: 
crT = a(T) + b(T)*S, where: 
T = temperature (°C) 
S = salinity (ppt) 
a and b are polynomial functions of T 
a(T) = -9.22x10 3 + 5.59x10 2 * T- 7.88x1 O' 3 * T 2 + 4.18xl0' 5 * T 3 
b(T) = 8.04x10-' - 2.92x10 3 * T + 3.12x10 5 * T 2 
3. Look down through the profile. Wherever the difference between sequential 
depth measurements is < 0.19 meters, average the two depth measurements and 
their corresponding salinity and density measurements. 
4. Look down through the profile again. If there are still any depths (depth, 
salinity, temperature and density measurements) <0.19 meters apart, then 
average them again. Continue until there are no depths <0.19 meters apart. 
5. Starting at the surface measurement and continuing until the deepest measure¬ 
ment in the profile, calculate the change in salinity and density between each 
sampling depth. For example, for two density values at 1 meter depth (y]) and 
2 meters depth (y 2 ) respectively, the change in density, or AcrT = y 2 -yj. Like¬ 
wise, for salinity measurements AS = y 2 -yj. 
6. Assign a depth measurement to each pair of A values (AS, AcrT) equal to the 
average of two depths x 2 and x, used to calculate the A values. Thus for the two 
measurements y 2 and y,, calculate the accompanying depth as (xj + x 2 )/2. You 
should now have a vertical profile of AS and AcrT values with an accom¬ 
panying depth. 
7. To find the upper boundary of the pycnocline, look at the vertical profile of 
AcrT, beginning with the second value (from the surface) and excluding the two 
deepest values: 
a. IF AcrT >0.1, 
b. AND IF AcrT for the next depth is greater than zero. 
chapter iii 
Refinements to Procedures for Assessing Chesapeake Bay Dissolved Oxygen Criteria 
