TMDL SAMPLING PROTOCOLS AND ASSOCIATED FIELD COLLECTION ISSUES: AN IOWA PERSPECTIVE

Iowa is an intensely agricultural state with the majority of its land in row crop production of corn and soybeans. The state's production of hogs and other livestock is also well documented. The environmental impact of agricultural land uses as well as urban point and nonpoint sources of contaminants has resulted in Iowa having 188 water bodies with 257 listed impairments on the 2002 303(d) list. These impairments are related to sediment and nutrients, with the majority of the impairing compounds entering streams and lakes during periods of rainfall and snowmelt runoff events. Therefore a monitoring strategy that includes baseline and event monitoring in impaired stream segments has been implemented to determine ambient concentrations and loading rates for the listed impairments.The monitoring strategy in Iowa is two-fold and a cooperative effort between the Iowa Department of Natural Resources and the University of Iowa Hygienic Laboratory. Monitoring consists of the collection of monthly grab samples to measure ambient concentrations of nutrients (nitrogen and phosphorus forms), various solids components, pesticides, bacteria and chlorophyll. Event monitoring, primarily for rainfall runoff is performed at the same locations as the ambient monitoring and completed each time stream discharge levels reach seasonal threshold values based on historic stream gauging information, where available. Field measurements for temperature, pH, dissolved oxygen and instantaneous flow are also taken at the time of sampling. The purpose of this presentation will be to discuss the comprehensive event monitoring strategies that have been employed to gain information on loading rates of contaminants.It is vital to the accuracy of a TMDL to collect data that is representative of the conditions of a water body over time during substantial loading events. Grab samples that provide “point in time” information are not comprehensively reflective of the impact of the event. The State of Iowa monitors stream events over a minimum of a 24-hour period of seasonally high flow using an automated composite sampler. Each sampler collects samples into 24 discrete sample bottles, with approximately 250 ml samples collected every 20 minutes throughout the 24-hour period. Corresponding stream stage heights are measured every 15 minutes using a differential transducer bubbler system attached to the composite sampler. The result of this sampling strategy is both target compound concentration data and calculated stream discharge values that, combined, provide loading information through the duration of the event. Loading information can then be sub-divided into “initial flush,” pre-peak, and post peak discharge loading rates for the event.This presentation will provide specific details on Iowa's network of samplers, how they are deployed, sampler programming considerations, collection methods, associated sampler software capabilities, and the advantages and pitfalls of using this type of automated system. Funding for this TMDL program is provided by the Iowa Department of Natural Resources.