Rates and Dates
An interactive introduction to environmental tracers — how water moves, where it came from, and how long it has been here. No isotope background needed.
A stream rises within minutes. How old is the water?
Rain begins over a forested catchment. Within minutes, the stream gauge shows discharge climbing. Choose the claim you would defend first.
When hydrologists first measured isotopes in storm runoff, the result up-ended the "obvious" answer A. In the classic storm analyzed by Sklash & Farvolden (1979), river discharge rose roughly fivefold (from ~25 to ~130 m³/s) — yet more than two-thirds of the water at peak flow was "pre-event" water, water already stored in the catchment before the rain began (Jasechko, 2019, Fig. 27). The rain arrived, and the catchment answered — largely with old water.
Be ready to name the observation that would change your mind — that habit is the seminar's standard of critique.
Hydrology measures motion; tracers reveal history
Conventional instruments answer "how much, how fast." Tracers answer where water came from, which route it took, how long it stayed, and what happened to it along the way.
What conventional observations establish
- Head and gradient describe hydraulic potential — which way water is being pushed.
- Discharge measures water moving past a section, right now.
- Storage change tracks the balance of gains and losses.
All three are indispensable — and all three are silent about the history of the molecules involved.
What tracer information adds
- Origin — which source supplied the water or solute?
- Pathway — which route connected source and sample?
- Age — when did the water enter the system?
- Process — what reactions or mixing changed it?
Four kinds of history, none of which a gauge can see.
An environmental tracer is information carried by water
A useful tracer has three properties. Miss any one of them, and the "tracer" tells you nothing you can defend.
1 · A structured signal
It varies across space, time, sources, or reactions. A constant carries no information.
2 · A transport story
It enters naturally or through human activity, and moves with — or predictably relative to — the water.
3 · An interpretable response
Its concentration or isotope ratio can be linked, through a model, to a hydrologic hypothesis.
Test the definition
Click each candidate and ask: does it have a structured signal, a transport story, and an interpretable response?
Pick a candidate above.
Two families of isotope tracers
Tracers of the water itself
Oxygen (¹⁶O, ¹⁷O, ¹⁸O) and hydrogen (¹H, ²H, ³H) isotopes are constituents of the water molecule — not something dissolved in it. In low-temperature systems the stable ones travel conservatively: waters keep their isotopic "fingerprints" until they mix with other waters (Kendall et al., 2014).
Tracers of the solutes in water
Isotopes of dissolved nitrogen, carbon, sulfur, and other elements fingerprint where a solute came from and what reactions transformed it — nitrate from fertilizer versus waste, carbon exchanged with soil or rock. Powerful, but rarely conservative: reactions shift the signal in predictable, recognizable directions.
Start with the question, not the isotope
Tracer choice begins with the hydrologic question — never with a favorite isotope or instrument. Five kinds of question cover most of hydrology. Open each to see a plausible first strategy.
Rates and dates answer different equations
RATE
How much water moves per unit time?
Recharge, discharge, exchange, velocity. Units: volume / time (or length/time).
DATE / TIMESCALE
When did water enter, and how long did it stay?
Recharge date, travel time, residence-time distribution. Units: time.
Try it: push rain through a hillslope
The column below is a hillslope's storage, drawn as stacked parcels of water — darker blue means the parcel has been in the ground longer. Press Rain storm and watch two different speeds at work: the discharge response (a pressure signal, fast) and the movement of the rainwater itself (slow). The stream responds immediately — but check whose water it is delivering.
Streamflow
parcels leaving per tick (relative discharge)
Share of streamflow that is this storm's rain
fraction of recently exited parcels that are event water
What just happened? Rain landing on the hillslope raises the water pressure all the way down the column almost instantly, squeezing the oldest stored water out into the stream — rainfall "pushing" pre-event water ahead of it. Hydrologists call the speed of that pressure signal celerity, and the speed of the water molecules themselves velocity (McDonnell & Beven, 2014). Fast response, old water: both can be true at once — and in real catchments, they usually are.
A water sample is usually a mixture of ages
A well or stream sample rarely has an age. It blends water from stores with very different histories, and a single reported "age" can collapse — and hide — that distribution.
Mix a sample yourself
Drag the sliders (or pick a preset) to set how much of a sample comes from each store. The bar shows the blend; the curve shows the age distribution it implies; the marker shows the single "mean age" that would be reported.
Shares are renormalized to total 100%.
The age distribution behind the sample
probability density of water age (note the logarithmic time axis)
View the numbers
Recent input
Event to seasonal. Highly responsive to current forcing — this is the water that carries this year's weather.
Active storage
Months to years. Mixes changing recharge histories; the buffer between weather and long-term memory.
Legacy storage
Years to millennia. Carries past climate and land-use signals — decisions made decades ago still flow out of it today.
The proportions depend on storage, connectivity, forcing, and the sampling moment: pumping, storms, season, and well-screen depth all change the mixture collected at one point. Week 3 will make this precise by separating apparent age, mean residence time, and travel-time distributions.
The bomb pulse turned the atmosphere into a time stamp
Tritium (³H) is radioactive hydrogen built into the water molecule itself, decaying with a half-life of 12.32 years. Atmospheric thermonuclear testing in the 1950s–60s spiked precipitation tritium to roughly 5–500× its natural background (which sat below ~15 TU over non-polar lands), peaking around 1963 — and then decay and atmospheric mixing brought it back down. That spike is a global time stamp written into every water body that received recharge.
Explore the pulse
The blue curve is precipitation tritium at a northern mid-latitude station (schematic, drawn to the pattern of IAEA network records — 60,859 measurements at 722 stations — synthesized in Jasechko, 2019, Fig. 6). Drag the slider to pick a recharge year: the dashed path shows that year's rain decaying to the present, and the aqua curve shows what rain from every year would measure if sampled today.
View the numbers
A tracer becomes evidence through a four-link chain
Between "we measured something" and "therefore the water is young" stand four links. Every link carries assumptions, and uncertainty compounds downstream. Click each link to see what can break there.
Click a link above to see its failure modes.
Global syntheses turn samples into testable hypotheses
A century of groundwater isotope measurements — compiled in Jasechko (2019) — now confirms and quantifies ideas that were once local hunches.
Recharge is biased toward cold and wet seasons
Aquifers fill disproportionately from winter and wet-season precipitation and from intense rainfall — not evenly from the annual total. Where recharge comes from matters as much as how much falls.
Modern groundwater is a small share of the total
Water replenished within recent decades makes up a considerable share of the shallowest ~100 m — but only a small share of groundwater in the upper 1 km. Aquifers flush far more slowly than management horizons assume.
Fossil groundwater dominates aquifer storage
Water recharged more than ~12,000 years ago — before the Holocene — dominates global aquifer storage, and its chemistry archives ice-age climate. Pumping it is closer to mining than to harvesting a renewable flow.
Groundwater generates much of streamflow
In 61 of 101 compiled storm studies, more than half of streamflow was groundwater or other pre-event water — including during many storms. Rivers are, to a first approximation, groundwater in motion.
Also confirmed: surface-borne contaminants are more common in younger groundwater — age and vulnerability travel together. Which of these findings is a rate claim, which is a date claim, and which requires both? That question returns in every week of this course.
Choose a tracer strategy
Four scenarios from the Week 1 activity. For each, pick the strategy you would try first — then compare with the debrief. There is rarely one correct tracer; there are more and less defensible inference chains.
Six questions before Week 2
Immediate feedback, no grade, no record. If you miss one, the linked section is the fix.
Glossary
The Week 1 working vocabulary, plus a few terms you will meet in Week 2. Search or browse.
The Week 1 readings — four complementary views
Papers are not posted for this course: retrieving them from the citation and DOI — via UGA Libraries, GALILEO, or the publisher — is part of the training.
Jasechko, S. (2019). Global isotope hydrogeology — review. Reviews of Geophysics, 57(3), 835–965.
The anchor overview: a synthesis of groundwater ²H, ¹⁸O, ³H, and ¹⁴C data, its major findings, and its geographic gaps. doi:10.1029/2018RG000627
Gat, J. R. (1996). Oxygen and hydrogen isotopes in the hydrologic cycle. Annu. Rev. Earth Planet. Sci., 24, 225–262.
The classic account of stable-isotope behavior across the whole water cycle. doi:10.1146/annurev.earth.24.1.225
Kendall, C., Doctor, D. H., & Young, M. B. (2014). Environmental isotope applications in hydrologic studies.
A broad map of water- and solute-isotope applications, mixing, reaction, and multi-tracer work. doi:10.1016/B978-0-08-095975-7.00510-6
McGuire, K. J., & McDonnell, J. J. (2015). Tracer advances in catchment hydrology.
A short, forward-looking perspective on age, origin, pathway, tracer-aided models, and emerging tracers. doi:10.1002/hyp.10740
Coming in Week 2 — Fundamental principles of tracer techniques
δ (delta) notation and isotope systematics · equilibrium and kinetic fractionation · radioactive decay · the global and local meteoric water lines and deuterium excess · conservative vs. reactive tracers. On the discussion table: Dansgaard (1964), Craig (1961), and Evaristo, Jasechko & McDonnell (2015) — reading notes begin.
Exit-ticket habit worth keeping: one hydrologic question you care about · one tracer you might use · one assumption you would have to test.