Prepared by the U.S. Geological Survey
in Cooperation with New Hampshire Department of Environmental Services
and U.S. Environmental Protection Agency

By Thor E. Smith

Current Schedule for River Assessments

As of October 11, 2000, time-of-travel assessments are scheduled for the following rivers during the months of October and November 2000.  The scheduled dye injection and sampling dates are subject to change due to streamflow and weather conditions, and the schedule shown here will be updated accordingly.  The methods of the study and a list of the river reaches to be studied are detailed in subsequent sections of this document.

                         River                          Study dates

              Contoocook                     Oct.  16 - 27
              Androscoggin                  Nov.  6 - 17
 
 

What are time-of-travel assessments?

Time-of-travel is a term used to identify assessments of transport rates for water flowing underground or at the surface. One objective of time-of-travel involves measuring the time it takes a dissolved (miscible) substance to flow through the earth or in a river from the point of injection to a location at a known distance away where water is withdrawn (e.g. a well or intake, respectively) for use. In addition, data collected in these assessments can be used to estimate the time it takes for a substance to spread or disperse over the width of a river and estimate the rate a substance is dispersed with distance from the point of injection.

Time-of-travel assessments here in New Hampshire

In cooperation with the State's Department of Environmental Services (NHDES), the USGS is conducting the time-of-travel assessments for 13 river reaches listed in table 1 below. For the assessments, a dye called Rhodamine WT will be the dissolved substance, as indicated in the previous paragraph, introduced to each river reach of interest from table 1. Results from the assessments will be utilized for mapping the reach of the river for which it would take the leading edge of a substance cloud or plume 6 hours to travel to the intake or withdrawal point for at least two discharges. Other results, which will be available, are concentrations of the substance with time, distance the substance traveled with time, travel-time with discharge, and concentrations of the substance with distance. Discharges for the analyses will be near the median or bank-full discharge, and near the discharge exceeded 90% of the time. Comparing the discharge values in table 1 below with the current discharges from the real-time data page will let you know if the discharge is at or near the discharge expected for the assessments.
 

Rhodamine WT is a flourescent red-pink liquid dye. The dye is intended to simulate a chemical spilled into the river as an experiment to observe how quickly a substance would spread-out and occupy the width of the river and how quickly a substance would travel from the location along the river where it was introduced to the location where water is withdrawn. In the assessment, dye is injected into the river water as an instantaneous mass or slug. The dye will quickly mix with the river water vertically, and will more slowly mix horizontally as the dye cloud flows downstream.

Once the dye is injected, personnel will move to a location at a known distance downstream of the injection point and prepare to begin sampling. At this first location, samples will be taken at three or more stations across the channel at regular intervals. The interval for sampling will vary depending on the slope or grade of the river, but may be on the order of once every 15 minutes to once every hour, until the dye cloud has passed. After the dye cloud passes, personnel will set-up an automatic sampling instrument at the location of the intake where water is withdrawn from the river.

Set-up of the automatic sampling instrument may occur the same day or a couple days after the dye injection depending on the slope of the river. Automatic sampling instruments will syphon up to 1 liter (about a quart) of river water on a regular interval (again anywhere from once every 15 minutes to once every hour may be applied) and are limited to a total of 24 samples. Personnel will visit the samplers daily. Samples will be removed and transported to our office laboratory for dye concentration measurements.

Our Mission, This Project, and You

As the nation's largest water, earth and biological science, and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This scientific information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation, economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources.