The availability of high quality data is arguably one of the greatest concerns and most important aspects in developing TMDLs and TMDL allocations. The amount and quality of data will often make or break the credibility of a TMDL and its potential for approval. Developers of TMDLs know
that the type, quality and quantity of data, as well as where they are collected in relation to the impaired waterbody, must be suitable before they are incorporated into a TMDL calculation. Vast quantities of such data are available from agencies such as EPA and USGS. However, the coincidence
of these datasets and their potential role in developing TMDLs on some 20,000 impaired waterbodies in the U.S. is largely uncertain. What is more, while many TMDLs have been developed and approved, data requirements for most TMDLs have not yet been adequately ascertained. The adequacy of
data used for calculating TMDLs is also dependent on several conditions of the available datasets including parameter type, period of record, sampling method, sampling frequency, and consistency. The collection of various chemical constituent observations for impaired surface waters is especially
important when modeling is considered for a TMDL. When modeling efforts are necessary, the ideal temporal record of the dataset should encompass several calendar years to represent a range of conditions. Gaps and inconsistencies in the record make it difficult for even experienced modelers
to calibrate water quality models for a given area. Other potential issues regarding data adequacy include the quality and reliability of the data. In order to quantify water quality datasets that are widely available for TMDL development, we randomly selected a 5% sample of hydrologic
units throughout the U.S. and evaluated each hydrologic unit for the availability of data potentially relevant to TMDL development at these locations. Datasets used in this assessment included retrievals from EPA's STORET Legacy Data Center (LDC) and the USGS National Water Information System
(NWIS). These datasets were also evaluated for their relevance in making site-specific TMDL determinations on known impaired waterbody segments within each hydrologic unit. Our study demonstrated that, with respect to these databases, many impairments contained no data. We believe that data
exist for at least some of these impairments and that these data should be available through these national repositories. This information will benefit decision makers and regulators by supporting concerns over water quality data availability heightened by rushed TMDL schedules that may force
states to make imprudent assessments of impaired waterbodies. Assessments made on such hastened schedules will likely impede the potential for TMDLs to effectively improve the conditions of our nation's surface waters.
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