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Synoptic Sampling of Surface Waters

Developing a spatial snapshot of water quality.

Background

Synoptic sampling employs three major components: synoptic sampling of water, constituent analysis, and mass balance calculations. As the Greek root of the term indicates, a synoptic sampling offers a summary of two key characteristics within a watershed at a singular time: streamflows and constituent concentrations. From these two characteristics, constituent loads at each sampling site can be easily calculated, and a mass balance across the watershed performed (Runkel 2013). If performed well, SM can provide valuable spatial profiles that point towards potential sinks and sources of the constituent of interest (Kimball et al. 2002). The most important distinction to remember about SM is the temporal nature of the approach–it’s only a snapshot of the hydrogeochemistry.

As one might imagine, synoptic sampling has been used extensively in many areas of environmental studies, particularly as a means to locate or prioritize sources of contaminants for remediation efforts. Numerous papers have been published by those studying acid drainage and toxic metals from mines, studies spurred on by the Abandoned Mine Lands Initiative (Kimball et al., 2007). Relatedly, synoptic sampling has been well developed and utilized through the U.S. Geological Survey’s Toxic Substance Hydrology Program (Runkel 2013).

One recent modification of the synoptic sampling approach comes from Runkel 2013. Likely due to the small extent of their study area (less than 1 mile in length), this team conducted back-to-back synoptic samplings, visiting 21 sites in the morning and revisiting them in the afternoon. Runkel argues that this immediate replication of the synoptic sampling allows for the placement of error bars on load estimates, aggregating all sources of uncertainty.

Another development in the history of synoptic sampling is the discovery and consideration of diel fluctuations of certain constituents. This is addressed later on.

How might one perform synoptic monitoring?

Having experienced synoptic sampling firsthand last summer with representatives from the Utah Department of Water Quality, I can verify the basic sampling procedure as described in many papers. Multiple sites are pre-selected to represent spatially the hydrologic nature of the particular watershed or subsection, accounting for streams and inflows to streams. Sites will often bracket an inflow, making useful the upstream and downstream mass balance. If targeting a suspected source, make sure to sample upstream as well as downstream, as was done by Cox 2013 when assessing the loads of phosphorus from a poultry farm. Sampling along the main stream, bracketing the inflows and so on, is an appropriate approach when the major concentration gradients exist in the longitudinal plane, rather than lateral (Kimball 2002).

All sites of interest need to be sampled as temporally close to each other as possible, usually within a few hours time. Due to the small extent of the study area in Runkel 2013, that team was able to sample all sites twice in one day. When sampling at the watershed scale, logistically it may only be possible to collect one set of samples within the course of a day. Time of year and flow conditions influence the specific approach toward synoptic sampling. Cox 2013 sampled during both spring and summer of 2005 and 2006, to capture high flow and baseflow conditions. Note here that in 2006, the sampling campaigns happened only three weeks apart.

In collecting the water sample from the stream, methodologies vary by either the nature of the constituent, by experience, or due to convenience. For example, one might obtain grab samples from the approximate center of streams, as done by Cox 2013. In the case of wider streams, Runkel 2013 made sure to integrate depth and width variability by using a DH-81 water sampler. Poor sampling methods would include, for example, sampling only at the stream bank or only at the water surface, locations where flow is not representative.

Another important consideration is the direction of travel between sampling sites. Those leading the synoptic sampling in the Wallsburg watershed last summer instructed us to proceed from upstream-to-downstream. This practice is questionable as upstream-to-downstream movements would increase suspended sediments in downstream samples. A downstream-to-upstream approach to sampling would eliminate the potential of contamination from resuspended streambed sediments (Runkel 2013). However, in a recent sampling event in Wallsburg, though we proceeded in a downstream direction, there was no apparent evidence for sediment contamination likely because of the low flow velocities (i.e. we moved faster than the water).

The constituent of interest will influence the synoptic sampling protocol used at a given site. For example, some acidification or preservation of the sample may be required, or perhaps immediate filtration may be necessary. Other water quality parameters may also need to be measured on site (pH, conductivity, etc.) (Runkel 2013).

While flow may often be measured with a portable device such as an acoustic doppler velocimeter, flow in high mountain streams is better measured using the tracer-dilution method (Kimball 2002). High mountain streams are often cobbly, may contain very shallow water, and some flow may be happening below the streambed in the hyporheic zone (Jarrett 1992). Tracer injection is a means to measure flow by which a chemical solution (e.g., lithium chloride) of known concentration is injected upstream and allowed to reach a steady concentration along the length of the sampling area. As the synoptic sampling is performed, the tracer will be collected in the water sample, and later the tracer will be analyzed in the lab along with the other constituents. The concentration of the tracer in the sample compared to the initial concentration provides flow information along the hydrologic network, signaling inflows and dilution or subsurface flow. The same tracer method could be applied to base flow conditions in any small stream network, increasing the amount of accuracy considerable. Another example of tracer-dilution comes from Runkel 2013, where the tracer proved invaluable to flow measurements since the stream hydrology was greatly complicated by beaver dams (Runkel 2013).

Samples are then analyzed in a laboratory using appropriate quality controls and standard procedures. Reducing laboratory error is paramount, especially when some constituents, such as dissolved phosphorus, may measure less than 10 ppb. Analysis of total element concentrations is commonly performed using inductively coupled plasma optical emission spectrometry.

Once flow calculations and lab analysis have been performed, the constituent loads can be calculated. The fundamental calculation of the mass balance is the simple product of the streamflow and the concentration at a given sampling location. This is the load. The sum of all loads at all sampling sites constitutes the estimated total minimum load (Kimball 2002). As was mentioned previously, Runkel 2013 performed replicate sampling events, such to provide four profiles of constituent load. These four profiles were derived by multiplying the two flow profiles against the two concentration profiles. Descriptive statistics and error bars were then calculated. Without Runkel specifying the purpose of this (to account for all error inherent in SM) this statistical footwork seems misdirected, as concentrations of certain constituents can fluctuate during a 12/24hr period. This cycling is mentioned more later.

Reasons for using synoptic monitoring

The strength of SM is the ability to identify point and nonpoint sources of pollution or water quality degradation. Although streamflows and concentrations of the constituent may be very different from sampling to sampling, by performing a synoptic sampling across a watershed in a few hours one gains a “spatially intensive ‘snapshot’” from which one can calculate loads, and then identify the highest loading inflows. The samples from two inflows may have the same concentration, but the inflow with the greater discharge will cause a greater increase in downstream concentration according to the load calculation (Kimball 2002).

To quantify the impacts of phosphorus from a local poultry farm, Cox 2013 conducted a synoptic sampling that bracketed the farm. The relationship between load and concentrations and discharge are manifest in the results.

Over short distances, the differences of phosphorus under nearly baseflow conditions was sevenfold, whereas during an elevated flow period the differences of concentration were nearly 20-fold (Cox 2013). These two SMB’s, conducted under different flow conditions allowed for Cox to conclude that poultry farming and its generated waste were the primary drivers of fluctuations in stream phosphorus concentrations.

Reasons to be cautious

The paramount assumption that fundamentally supports synoptic mass balance is that the sampling event is conducted under steady flow conditions, constituent concentrations also being constant for the duration of the event (Runkel 2013). For example, to perform a sampling event during the receding limb of a storm event would be disastrous. Instead, many sampling events at higher elevations are conducted toward the latter end of summer, when snowmelt is negligible. Aside from the steady state conditions surrounding low-flow or base-flow, these are desirable sampling times as it often corresponds to higher concentrations of the constituent of study (Grayson et al 1997).

Relatedly, one must also be cautious with the potential for concentration fluctuations even on the 12/24hr time period. Scientists often reasonably assume that a properly collected water sample will provide an accurate assessment of constituent concentrations (USGS 2014). However, the concentration of trace elements (such as arsenic, cadmium, zinc, etc.) in streams can vary over a large range during a 24-hour period. Irrespective of changes in streamflow, these diel cycles have been shown to be “robust and reproducible, having been documented in many streams separated by large distances, in different geologic environments, and over a large range of metal concentrations” (USGS 2014).

Another area of caution to take is in the analysis and interpretation of concentrations. For instance, analyses should be conducted on the dissolved fractions of conservative constituents to avoid sampling artifacts. On the other hand, reactive constituents should be analyzed for total recoverable concentrations, such that loads are not underestimated (Runkel 2013). In my own study, we have chosen to measure both, and so this mentioned problem is not so important. On the topic of analysis, laboratory errors can lead to uncertainty, both in constituent concentrations and, if used, tracer-dilution estimates (Runkel 2013). The margin of error could be potentially large as some constituents are measured in concentrations as low as a few parts per billion.

One must also understand potential complexities in the local hydrology that could affect the accuracy of the tracer-dilution method of flow measuring. Significant amounts of tracer could be lost to extended subsurface flow paths, not re-entering the stream over the time scale of interest (Payn et al 2009).


References

Cox, T.J., et al. (2013) Relationships between stream phosphorus concentrations and drainage basin characteristics in a watershed with poultry farming. Nutrient Cycling in Agroecosystems, 95(3):353-364.

Grayson, R.B. et al (1997) Catchment-wide impacts on water quality: the use of “snapshot” sampling during stable flow. Journal of Hydrology, 199: 121-134.

Jarrett, R.D. (1992) Hydraulics in mountain rivers. International Conference, Littleton, CO. 287-298 Water Resources Publications.

Kimball, B.A., et al. (2002) Assessment of metal loads in watersheds affected by acid mine drainage by using tracer injection and synoptic sampling: Cement Creek, Colorado, USA. Applied Geochemistry, 17(9):1138-1207.

Payn, R.A. et al (2009) Channel water balance and exchange with subsurface flow along a mountain headwater stream in Montana, USA. Water Resources, Res. 45, W11427.

Runkel, R.L., et al. (2013) Estimating instream constituent loads using replicate synoptic sampling, Peru Creek, Colorado. Journal of Hydrology, 489:26-41.

USGS 2014. Diel Cycling of Trace Metals in Streams. Last Modified June 2014. Retrieved from: http://wy-mt.water.usgs.gov/projects/diel_cycling/

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