Flume installation and preparation for monitoring: Part 1

This portion of the construction and installation process for the P removal structure is NOT necessary for the typical-everyday use.  What is described here is for very detailed monitoring of performance for the purpose of research.  It is completely unnecessary for the applied purpose of the P removal structure.  We are doing this so that we can demonstrate the effectiveness of the unit and also produce more data for model verification.  This section is mostly relevant to researchers.

That being said, we will not put the PSM (treated steel slag in this case) into the structure until all the monitoring equipment is installed.  For monitoring, our purpose it to measure the amount (volume) of runoff treated by the structure during each runoff event and also measure the P concentrations flowing in (i.e. before treatment) and after (i.e. after treatment).  Knowing this, we can then estimate how much (mass) P has been trapped by the P removal structure. 

On the “inflow” side of the structure where water will enter, we will use an ISCO automatic sampler that will be triggered to collect samples based on the detection of flow at the “outflow” side of the structure where the treated water will be located.  Below you can see some photos of the construction of the approach for the 3 foot flume that will be used to monitor flow rate.  

Monitoring flow rate is critical if one is serious about getting a real estimate on the P removal performance of a P removal structure.  Many researchers have utilized only water sampling in their monitoring regime without the use of flow monitoring.  Simply put, P concentration testing of the treated water compared to untreated water alone is not sufficient for assessing the capacity of a P removal structure; knowledge of the flow rates and therefore the volumes of water treated is absolutely critical.  Any assessment without the use of detailed and thorough flow monitoring should not be taken too seriously.  Why? The short answer is “P load”.  If I measure a 100% P concentration reduction and there is only 1000 gallons of water treated, it is not correct to compare that concentration reduction to another scenario where there is only 50% P concentration reduction and 1 million gallons of water treated.  Simply put, the most important factor is the P load (i.e. mass) reduction.  Instead, it is more correct to compare the P load reduction (i.e. mass of P removed).

Too often, we focus on the final concentration of runoff water and treated water.  This makes little sense, especially when surface water quality thresholds (i.e. critical P concentrations for streams and lakes) are applied to the context of runoff water concentrations.  For example, runoff concentrations from a certain field (call it field 1) may test at 1 mg P/L, while field 2 may produce 0.2 mg P/L.  That does not mean that field 2 is somehow more “safe” than field 1.  Consider a scenario where field 1 produces 2 million L runoff for an event, while field 1 only produces 1000 L.  The P load transported off site for field 1 and 2 would be 1 g vs 400 g, respectively.  Concentrations alone would be deceiving in that case.  What matters is the mass or load of P that reaches the lake or stream; concentrations (mass/volume) change with dilution, evaporation, etc., but it is the mass of P that does not change.  This is why the USEPA has moved to a “total maximum daily loading”, or TMDL system for point and non-point source pollution concerning nutrients. 

For the same reasons, it is important to assess the performance of a P removal structure based on P load reduction, not P concentration reduction. 

Below you can see the flume installation process.  

This flume can handle much more water than our projected 2 yr storm (~16 cfs).  This flume is being used courtesy of Dr. Sherry Hunt, Kem Kadavy, and Ron Tejral, located at the USDA-ARS Hydraulics Engineering Research Unit.

All of the treated water plus any water that might overflow the structure will flow through the flume for measurement.  There will not be any water overflowing the structure if none of the storms exceed a 2 yr return period for that location.  In addition, if that does happen, there will be a flow sensor (actually a depth sensor) placed on top of the structure that will also monitor exactly how much water overflows the structure and remains untreated.  Below is the view looking from the structure downhill toward the flume:

This area between the outflow drainage of the structure and the flume will have an impermeable liner placed on the ground and “bordered” with stabilized railroad ties to force all water from the structure to flow through the flume where flow rate will be monitored and also where samples will be taken by the ISCO sampler. 

The photos below show the installation of the small building that will house the ISCO sampling equipment: