[L&O Featured Article]Vol. 48, Issue 1, January 2003

[lo-feature@aslo.org]

Current Featured Article

The Featured Article in the January 2003 issue of L&O is:

James M. Krest and Judson W. Harvey. 2003. Using natural 
distributions of short-lived radium isotopes to quantify groundwater 
discharge and recharge.  Limnology and Oceanography 48(1) 290-298.

This paper is freely available at this Web address:

          http://aslo.org/lo/toc/vol_48/issue_1/0290.pdf

Instructions for reading PDF files are located on the ASLO web page: 

          http://aslo.org/help/loonline.html 

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Introductory Comments by Don Siegel

How groundwater interacts with wetlands significantly controls their 
biogeochemical cycling, and ecological evolution and health (e.g., 
Winter and others 1998; Glaser and others 1991).  For example, 
groundwater discharge (upwelling of water) to wetlands dominated by 
sedges provides water to saturate wetland soils and solutes and 
nutrients needed to maintain unique vegetation communities (Bedford 
and others 1999). In contrast, groundwater recharge (downward flow) 
under raised bogs transports bio-available organic matter from plant 
roots downward to methanogens and other bacterial populations deep in 
the wetland peat (Chasar and others 2001). Unfortunately, measuring 
the extent to which ground water discharges or recharges to wetlands 
is not a trivial matter. 

Many investigators use small diameter special tubes called mini-
piezometers to measure changes in hydraulic head (water levels) with 
depth in wetland soils to assess the recharge-discharge "function," 
but data from these installations can be compromised by leakage along 
the sides of the well casing, episodic overpressuring caused by 
methane and carbon dioxide accumulations (Siegel and others 2001), 
and pressure pulses caused by transient resaturation or drying at the 
top of the peat column (Waddington and Roulet 1996).   Whether a 
component of groundwater moves up or down in wetlands also can 
seasonally change, reducing the value of a few synoptic measurements 
of hydraulic head to evaluate the process.  Geochemists have used 
isotopic measurements of oxygen and hydrogen in water, carbon in 
dissolved inorganic carbon, and strontium in dissolved strontium to 
evaluate the long-term, or "quasi-steady state" extent to which 
upwelling ground water mixes with near surface precipitation recharge 
in wetland peat profiles (e.g., Hogan and others 2001; Chasar and 
others 2001). However, in-situ geochemical and biochemical processes 
can sometimes obscure clear discrimination of the water types.  

In the featured paper of the current issue of L&O, Krest and Harvey 
(2003) show how groundwater discharge and recharge in freshwater 
wetlands can be identified and quantified from the natural 
distributions of short-lived radium isotopes.  Basically, the method 
relies on comparing differences in radium isotope activity in peat 
pore water with the radium activity predicted from local production, 
decay and exchange processes. Radium isotopes are formed at different 
rates in wetland surface waters, peat pore water, and underlying 
mineral soils. Therefore, radium isotopes turn out to be excellent 
environmental tracers for groundwater flow.  In their case study, 
Krest and Harvey effectively used radium isotopic profiles in the 
Florida Everglades peat pore water to evaluate groundwater recharge 
and discharge. Groundwater hydraulics in this peatland are 
notoriously difficult to evaluate because of very small groundwater 
flow velocities, and the success of the radium method in this setting 
is all the more impressive.

The potential of using radium isotopes to assess the groundwater-
recharge function in wetlands is very great, and I expect that this 
paper will lead to many other applications of the method in the near 
future. 
  

References 


Bedford, B. L., M. R. Walbridge, and A. Aldous. 1999. Patterns in 
nutrient availability and plant diversity of temperate North American 
wetlands. Ecology 80: 2151-2169.

Chasar, L. S., J. P. Chanton, P. H. Glaser, D. I. Siegel, and J. S. 
Rivers. 2001. Radiocarbon and stable carbon isotopic evidence for  
transport and transformation of dissolved organic carbon,  dissolved 
inorganic carbon, and CH4  in a northern  Minnesota peatland. Global 
Biogeochemical Cycles 14: 1095-1108.

Glaser, P. H., J. A. Janssens, and D. I. Siegel. 1991. Response of 
vegetation to hydrological and chemical gradients in the Lost River 
Peatland, northern Minnesota. Journal of Ecology 78: 1021-1048.

Hogan, J. F., J. D. Blum, D. I. Siegel, and P. H. Glaser. 2000. 87Sr/ 
86Sr as a tracer of groundwater discharge and precipitation recharge 
in the Glacial Lake Agassiz Peatlands, Northern Minnesota, USA. Water 
Resources Research. 36: 3701-3711. 

Siegel, D. I., J. P. Chanton, P. H. Glaser, and D. O. Rosenberry. 
2001. Estimating methane production rates in bogs and landfills by 
deuterium enrichment of pore-water. Global Biogeochemical Cycles 15: 
967-977. 

Waddington, J. M., and N. T. Roulet. 1997. Groundwater flow and 
dissolved carbon movement in a boreal peatland. Journal of Hydrology 
191: 122-138.

Winter, T. C., J. W. Harvey, O .L. Franke, and W. M Alley. 1998. 
Groundwater and Surface water—a single resource. USGS Circular 1139, 
79p.

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