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WHAT IS AN ENVIRONMENTAL TRACER? Dissolved constituents, isotopes, particles or physical properties of water that are used to track the movement of water through watersheds
Source: USGS circular 1139http://pubs.usgs.gov/circ/circ1139/
POTENTIAL CHARACTERISTICS OF TRACERS
naturally occurring (e.g. chloride, silica, stable isotopes, organic compounds)
artificial or researcher introduced (e.g. various dyes, plastic microspheres)
sometimes unintentionally introduced! (e.g. tritium, chlorofluorocarbons, certain radioactive isotopes)
a less common isotopic form of an element physical property of water (e.g. temperature)
USES OF TRACERSGeneral: Used to identify flow paths, travel times, etc. Specific uses:
Subsurface processes e.g. preferential flow, groundwater movement
Surface processes Biogeochemical interactions
e.g. biological nitrogen uptake
Image source: http://www.twp.west-bloomfield.mi.us
sedimentpollutants
nutrients
ISOTOPES Same # of protons and electrons; different #
neutrons, so different masses! Some isotopes not very dominant
ISOTOPIC COMPOSITION Represented as ‘delta’ or ‘per mil’ δ(in ‰) = (Rsample/Rstandard - 1)1000
where "R" is the ratio of the heavy to light isotope in the sample or standard
A positive δ value means that the sample contains more of the heavy isotope than the standard; a negative δ value means that the sample contains less of the heavy isotope than the standard
ISOTOPE FRACTIONATION Fractionation: when the relative
amounts of a particular isotope change due to the mass differences ie: lighter H & O isotopes are preferentially
evaporated Equilibrium vs Kinetic fractionation
Equilibrium: redistribution occurs, but reaction rates same for forward/backward direction
Kinetic: reaction rates not same if products become isolated from reactants
DYE AS A TRACER Useful in surface and groundwater
studies In subsurface, useful for investigating
infiltration patterns, flow patterns for contaminants
In streams, useful for quickly evaluating travel time & mixing
STREAM TRACERS Types: Conservative
Don’t react biologically or strongly sorb to sediment
ie: bromide, chloride Reactive
Compounds affected by biological and physical reactions
ie: NO3-
STUDYING SUBSURFACE INTERACTIONS Hyporheic flow from woody debris
sediment
stream
FLOW
SUBSURFACE FLOW
DNA MICROSPHERES
Polylactic acid forms the framework
Magnetic iron oxide nanoparticles to enable capture
DNA for identification (and ability to have multiple “tags”)
DNA: a polymer of four types of monomers (A, T, C, G)Tracer of length m:X1 X2 X3X4 …Xm Xi = {A, T, C, G}Number of potential tracers = 4m
PLOT EXPERIMENT: RESULTS
Simple modified one dimensional advection dispersion model with a dispersion coefficient of 0.005 m2/s and a loss factor of 6.6:
Tracer 1
Tracer 2
BIOMARKERS FOR TRACING P Excess phosphorus applied as fertilizer
can end up in streams and lakes in the watershed
Phosphorus can be sorbed on sediment on on colloidal (<0.45 um ) particles/ dissolved in water
If we want to know where the P is coming from….sediment tracing works for P sorbed on sediment, but what about dissolved P?
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