Hydrochemistry of Forested CatchmentsM. Robbins Church

• Defines hydrochemistry and provides an overall introduction to Catchment Hydrochemistry

• Discusses the processes and factors that may influence the fate, transport, and exports of solutes from catchments

• Approaches to studying watershed hydrochemistry

What is Hydrochemistry?

Study of evolution of water or runoff chemistry in catchments and the processes and factors that influence the fate and transport.

What are the key factors that dictate the fate, transport and exports of solutes from catchments?

• Stores/Pools of solutes in the catchment and their location

• Hydrologic flow paths and their intersection with these pools

• Physical/Chemical and biological processes that regulate the pools

Figure 1 – conceptualizes the processes, hydrologic flow paths and transport mechanisms for solutes

Hydrologic flow paths and processes

Key factors that influence the runoff chemistry along the hydrologic flow paths –

• Chemical composition of precipitation• Abiotic materials or biota that are intersected by flow paths• Reactivity of solutes• Contact time of water and solutes

• Knowledge of flow paths and the contact time provide an idea of how watersheds “work”

• Flow paths may vary dramatically between baseflow and storm event periods

• Totally new sets of flow paths may be “activated” during storm events

Runoff in catchments can be generated by –• Direct interception of runoff by the stream channel• Surface flow• Subsurface flow• Groundwater

The flow paths and the amounts of runoff generated via the various mechanisms will be influenced by – precipitation characteristics, topography, geology, soils, and vegetation type.

Concepts of Old (pre-event) and New (event) water

Old water (pre-event) – water residing in the catchment before the storm event – has a greater contact time – will have a different chemistry

New water (event) – water introduced by precipitation – less contact time

• The relative amounts of these waters in streamflow will have a profound influence on stream water chemistry

• Interestingly, most studies show that the runoff that comes out during storms is primarily old water!

• So the key question is – how does the old water come out?

• Many theories explaining the quick discharge of old water from catchments

Various methods have been used to characterize the sources and origins of runoff – Spatial or temporal origins

Techniques that have been used range from –

• Naturally-occurring chemical tracers

• Naturally occurring Stable isotopes (especially 18O and 2H)

• Other methods – such as numerical of graphical methods for hydrograph separations

Chemical and Biological Processes

In addition to flow paths, it’s critical to know how the pools of solutes may change with time and the processes that affect these pools

Key processes –

• Physical processes of erosion and gas exchange• Chemical processes of weathering, chemical precipitation, cation

exchange, and ion sorption• Biological processes of uptake, respiration, decomposition,

mineralization, oxidation and reduction

These processes will continuously modify the pools at various time scales –annual and seasonal, event.

The article highlights key examples where such processes have come into play --

I Acid Deposition and its influence on base cation leaching from soils

pH of natural waters (without acidic ions)?

NOx and SOx react with water to form acids

So what problems do these acids cause?

What type of soil?What are the horizons?What is the white horizon?

Displacement of base cations off soil exchange sites by acid rain.

So how does this affect forest growth and vitality?

• Shallow flow paths • Less contact time • Low cations• Low pH – closer to rainfall

• Deeper flow paths, • Greater contact time • More cations• Higher pH Hydrologic flow paths pay an important role in

catchment response – buffering acid rain

II Forest growth and vegetative uptake on Nitrogen cycling and saturation in forests

Relates to the amount of NO3 that is available to be leached in catchment runoff

Inputs of N – atmospheric, anthropogenic, internal cycling

The consumption/sequestration/removal of N in catchments is influenced by two key processes • Vegetative uptake• Denitrification

If plants/forests are removed or if forest reach a stage of maturation – N consumption will decline. This will allow excess N (NO3) to accumulate in catchments and which will be leached with runoff.

There may be seasonal variation too in NO3 exports that are driven by seasonal patterns of forest growth!

II Forest growth and vegetative uptake on Nitrogen cycling and saturation in forests

There may be seasonal variation too in NO3 exports that are driven by seasonal patterns of forest growth!

dormant growing

At Stage 0, nitrogen transformations aredominated by plant and microbial assimilation (uptake), with little or no NO3 leakage from the watershed during the growing season.

Small amounts of NO3 may run off during snowmelt, producing the typical Stage 0 seasonal NO3 pattern.

Data in lower panel are from Black Pond, Adirondack Mountains.

STAGE 1 of watershed nitrogen loss. (Top panel)As in Stage 0, uptake dominates the nitrogen cycle during the growing season at Stage 1 and little or no NO3 leaks from the watershed during the summer and fall. The primary difference between Stage 0 and Stage 1 is the delay in the onset of N limitation during the spring season.

(Bottom panel) Large runoff events (e.g. snowmeltor rainstorms) during the dormant season can produce episodic pulses of high NO3 concentrations,as shown in the typical Stage 1 seasonal NO3 cycle.

Data in bottom panel are from Constable Pondin the Adirondack Mountains.

(c) STAGE 2 of watershed nitrogen loss. (Top panel) Uptake of nitrogen by forest plants and microbes is much reduced at Stage 2, resulting in loss of NO3 to streams during winter and spring and to groundwater during the growing season. Loss of gaseous forms of nitrogen through denitrification may also be elevated at Stage 2 if conditions necessary for denitrification are present. Although episodes of higher NO−3 concentrations continue to occur during high-flow events suchas spring snowmelt, the primary difference between Stage 1 and Stage 2 is the presence of elevated NO3 concentrations in groundwater. (Bottom panel) The typical seasonal NO3 pattern at Stage 2 includes both high episodic concentrations and high base-flow concentrations. Data in bottom panel are from Fernow Experimental Forest, Control Watershed No. 4, West Virginia.

At Stage 3, no sinks for nitrogen exist in the watershed and all inputs, as well as mineralized nitrogen, are lost from the system either through denitrification or in runoff water.

Because mineralization supplies nitrogen in excess ofdeposition, concentrations of NO3 in runoff may exceed those in deposition.

(Bottom panel)Typical seasonal NO3 pattern at Stage 3 includes concentrations at all seasons in excess of concentrations attributable to deposition and evapotranspiration.

Data are from Dicke Bramke in Germany andrevised from Stoddard (1994)

Approaches to studying Catchment Hydrochemistry

Small Catchment Approach

• Small catchments < 500 ha

• Very popular approach to understanding watershed functions and workings

• Numerous studies across the world

• Pioneered by studies at Hubbard Brook and Coweeta Hydrologic Laboratory

• Convenient and manageable in scale

• The small size allows for assumptions to be made

• Some of the assumptions that have been made may not be correct – “closed” systems

Coweeta Hydrologic Laboratory http://coweeta.uga.edu/Located in the Blue Ridge Physiographic province of North Carolina2185 hectaresStreamflow monitoring in 1934Stream chemistry monitoring – 1968

PI – Dr. Wayne Swank

Hubbard Brook Experimental Forest (HBEF)http://www.hubbardbrook.org/Established in 1955 in the White Mountains of New Hampshire3307 ha watershedStream chemistry monitoring started in 1963First watershed where budgets for element cycling were developed Dr. Gene E. Likens

Catchment Monitoring

• Measuring the response of catchments – small or large

• Important insights into how watershed are behaving and responding to external influences can be derived from studying watershed data

• This data can be long-term or short-term

• This data can be used to – test hypotheses, develop conceptual and numerical models of watershed functioning

• Critical that catchment monitoring should be driven or geared towards addressing specific questions, OR should be designed with an end goal in mind.

• Examples of lessons learnt from catchment monitoring!

Figure 2: Stream water concentrations (monthly average) of nitrate in four forested northeastern catchments before and after a period of unusual cold and soil freezing.

Catchment manipulations

• Direct experimentation – e.g., paired watershed studies• Direct evaluation of hypotheses• Typically involves watershed of similar characteristics and response• Manipulations have involved – forest removal, forest practices, fertilizer or

chemical additions

Example of the Bear Brook Watershed Manipulation project in Maine –http://www.umaine.edu/drsoils/bbwm/AboutBBWM.htmhttp://www.umaine.edu/drsoils/bbwm/Treatment.htmIntent – investigate catchment response to input of acidic or acidifying substances

• Two 10 ha paired watersheds were selected which were similar in hydrologic and biogeochemical response

• West Bear Brook – dry ammonium sulfate was added – applied bimonthly at the rate of 1800 equivalents per ha per year

• Tripled the sulfur loadings and quadrupled the annual loadings of nitrogen

Example of the Bear Brook Watershed Manipulation project in Maine –

• Stream concentrations of treated watershed increased dramatically

• Total N output doubled, mainly due to increases in NO3

• Responses indicate – that increase in atmospheric inputs can have a dramatic and rapid effect on stream chemistry

• The responses were consistent with accepted conceptual models

• Losses of N during the summer growing season indicated that N had reached deeper soil horizons and was being exported via deeper flow paths

Figure 4: Streamwaterconcentrations of nitrate, sulfate, calcium, and total aluminum in East Bear Brook (reference catchment) and West Bear Brook (manipulated catchment) before and after additions of ammonium sulfate to the soils of West Bear Brook.

Use of Catchment Models

• Useful tool - Especially useful for future long-term predictions – e.g., determining how catchments will respond to decrease in acid deposition

• Models can vary in their complexity

• Need to be careful – since predictions could be incorrect or uncertain

• Various types of models with different purposes and philosophies – EMMA, TOPMODEL, PnET-BGC, etc..