24 
1.5 root (Appendix D). The regression equations in Table IV-2 provide regional 
groupings and their regionally appropriate coefficients. 
Note that the equations in Table IV-2 represent regions that pertain to a subset (30) 
of the 92 Chesapeake Bay assessment segments. These equations were developed 
with the best available shallow-water monitoring data throughout the Chesapeake 
Bay. As data becomes available with future monitoring applied to other segments, 
the specific groupings and their respective equations can be expected to change in 
the future as a result of new data from the unassessed regions. 
INTERPOLATION SOFTWARE AND APPROACH 
Monthly shallow water monitoring dataflow data can be imported into ArcGIS 9.2 
(ESRI 2007) map visualization software as a point dataset or as a layer in ESRI’s 
ArcMap Geostatistical Analyst Extension. A single point dataset consists of a single 
DATAFLOW cruise, typically representing a single Chesapeake Bay segment. Each 
point in the dataset has an associated measured value for chlorophyll, dissolved 
oxygen, pH, salinity, temperature, and turbidity. A cruise track typically contains 
3000-5000 points with a range of approximately 2500-6000 georeferenced locations. 
The data are generally collected from April through October with 1-2 cruises per 
month. Within a cruise dataset, duplicate data values for a georeferenced point in 
time are averaged. This is important for Arclnfo because in the present Arclnfo 
workstation environment when kriging is conducted, Arclnfo cannot work with 
duplicate points. However, kriging conducted in ArcMap’s Geostatistical Analyst has 
the capacity to deal with duplicate data and the same step is not necessary. Missing 
data are provided with an error code (e.g., Virginia uses a value of -999). 
As previously documented in Table IV-1, for the attainment assessment, U.S. EPA 
(2007, pp. 54-55) indicated “Calculation of water clarity acres should be based on 
spatially intensive shallow-water monitoring turbidity data converted to K d ”, but the 
discussion further indicates “interpolated as described in Chapter 2, and then 
compared to the corresponding K d threshold assigned to each interpolator grid cell”. 
Chapter 2 (U.S. EPA 2007 p.l 1) provided a step-by-step approach to how the inter¬ 
polation would proceed if only a single parameter is involved in the assessment (e.g., 
dissolved oxygen for dissolved oxygen attainment measures). However, turbidity is 
not equivalent to or directly translated into K d . The regionally-specific multiple 
regression model approach (see Table IV-2) requires additional steps to get from 
water quality measure to threshold assessment for attainment or impairment. 
Details of the water clarity assessment framework, including a step-by-step approach 
to assessing attainment, are provided in Appendix E. Appendix F shows 2008 Mary¬ 
land and Virginia 303d/305b Chesapeake Bay water clarity assessment results to 
provide examples of water quality criteria attainment assessment output. 
chapter iv 
Refinements to Procedures for Assessing Chesapeake Bay Water Clarity and SAV Criteria 
