C-3 
a statistical basis for decision-making using the CFD (see pages 164-165 in U.S. 
EPA 2003a). Such a basis would allow the incorporation of error analysis into the 
criteria attainment assessment methodology. It would also allow the differentiation 
of an assessment based on a well-characterized system from one that was poorly 
characterized. Estimates of interpolation error are important to develop such a statis¬ 
tical framework. Such estimates allow decision-making to be based on the number 
(density) of sampling locations and promote greater statistical certainty (i.e., greater 
sampling density) in the assessment. The current Chesapeake Bay interpolation algo¬ 
rithm does not yield spatial error estimates (Appendix D); however, kriging is a 
possible alternative algorithm that can provide spatial interpolation error (STAC 
2006). 
Chesapeake Bay spatial interpolation requires the potential for automation. For 
many reasons, the Chesapeake Bay Program must compute many interpolations 
quickly. In developing the attainment figures for the 2006 listing cycle, for example, 
the program performed a total of 2328 interpolations for the final criteria assessment 
analysis of the 95 water quality segments). During development of the methodology, 
these interpolations were carried out repeatedly. Also, water quality models are often 
used to evaluate the potential benefits of management actions with the generation of 
multiple scenarios. Management action success is often defined in terms of water 
quality criteria, with results evaluated similarly to the actual measurements. Given 
the large number of data sets, automating the criteria assessment methodology and 
spatial interpolations would likely prove necessary. The current Chesapeake Bay 
interpolator allows automation and has been used in this way (Appendix D). Kriging, 
however, is a more detailed analysis that requires multiple decisions along the way, 
is not conducive to automation, and may not necessarily remain consistent within 
and between jurisdictions. 
DATA USED TO ASSESS CHESAPEAKE BAY 
WATER QUALITY CRITERIA 
As stated, the Chesapeake Bay Program redesigned the tidal monitoring program 
specifically to support water quality criteria assessment. That redesign resulted in 
multiple monitoring program components, all of which address one or more of the 
objectives of the Chesapeake Bay Water Quality Monitoring Program. Two of the 
components that serve most of the current needs of criteria assessment include the 
Baywide Fixed-station Water Quality Monitoring Program and the Shallow-water 
Monitoring Program. These two long-term efforts will provide data useful at 
different scales. 
The fixed-station monitoring program began in the mid 1980s and was designed to 
provide data for assessing long-term trends at key sites throughout the Chesapeake 
Bay and its tidal tributaries (Chesapeake Bay Program 1989). The program collects 
water quality samples at more than 150 sites (Figure C-l), including 49 stations in 
the mainstem Chesapeake Bay and 96 stations in the tidal tributaries. The samples 
go to a network of laboratories for analysis, compiling data on 19 water quality 
parameters. Fixed-station monitoring cruises run on a monthly basis throughout 
appendix c 
Evaluation of Options for Spatial Interpolation 
