Review of URS Corporation Analysis of Two Documents Provided by North 

Carolina State University

By

Richard A. McLaughlin, Ph.D.

Professor and Extension Specialist

Department of Soil Science

North Carolina State University

December 6, 2010

Summary

The following is a review of a Section Seven of the URS Corporation analysis of the 

development of the EPA Effluent Limit Guideline1

. Section seven was an analysis of 

three “true” Passive Treatment Systems (PTS) for which information or data were

available for use in the ELG calculation. The purpose of this review of that analysis is

primarily to correct misinterpretations of data cited in this document which were from

two studies conducted by my group. The misinterpretations revolved around what data 

were included or not and why. Where data are missing for certain dates in tables, it is 

because the sampler failed to obtain a sample and no data were available, not because 

the PTS failed. The only data that were omitted was for samples collected during 

periods when the PTS was compromised or bypassed as a result of site activities, as 

observed upon sample collection soon after the storm event. Data were omitted 

because they represented poor PTS management, not the capabilities of PTS. To 

facilitate a review of these misinterpretations, the relevant section (Seven) was pasted 

into this document and my comments, in italics, were inserted directly after the 

erroneous section.

I do not have the data from the third PTS cited (“Morrisville”) in the URS analysis, 

however I did visit that site and saw at least some of the remnants of that attempt at 

turbidity reduction. In my opinion, the failure of the PTS was most likely due to improper 

installation. It appeared that solid flocculants were placed in open, rock-lined ditches 

where they likely had uneven interaction with the runoff and were likely inactivated 

through exposure to the elements between storm events. This may have been the 

typical “afterthought” approach to turbidity control, rather than integrating PTS into the 

overall stormwater management plan. It was not surprising that there was little or no 

turbidity reduction.

1

URS Corporation, “Discussion of the EPA Calculation and Implementation of a 280 NTU NEL 

Limit Based on Passive Treatment Technology for the Final C&D ELG,” March 2010. Accessed 

at http://newsletters.wetlandstudies.com/docUpload/PassiveTreatmentCD_ELG.pdf.
In addition, I have added a table of data from a 3rd site that was very similar to the sites 

mentioned as “NCR.1” and NCR.2”, which correspond to Site 1 and Site 2 in the 

publication (McLaughlin et al., 2009), in Attachment 1. This data is included to further 

illustrate the potential for successful application of PTS. It was not included in the 

publication because we had few events in which samples were collected from all 

experimental sections of the site, due primarily to automatic sampler issues or low flow. 

I have included some very recent data as well from a site we are currently monitoring 

(Attachment 2).

With properly installed PTS, we have had good success in reducing turbidity by 1-2 

orders of magnitude in both controlled field testing and on active construction sites. 

When this does not occur there is almost always a problem with either the installation or 

maintenance of the PTS or activity that compromises the PTS.
(The following is copied from the URS document cited above)

VII.A The Data from the 3 True PTSs were Intensively Screened and Edited

by the Authors, yet EPA Undertook Further Editing of Very Limited

Data

Section Summary: The three true examples of PTS included in the limit calculations had

very few data points, and were already heavily edited by the authors of the papers to only

present the most favorable results. Nevertheless, EPA removed an additional point from

the NC-RD site for no other reason than the result was higher than the only three other

average results available. This removal was inappropriate, because it was a major

proportion of the available data, and the authors could not cite a specific problem that

would invalidate this data point.

The three true PTS systems used by EPA in the NEL calculations were all installed as

experimental BMPs at road construction sites in North Carolina by Professor John McLaughlin

and his associates from the University of North Carolina. 

Correction: Professor Richard A. McLaughlin, North Carolina State University

URS believes that these BMPs are a useful improvement over standard BMPs on construction 

sites. Where URS has a disagreement with Dr. McLaughlin is as to whether these BMPs can 

consistently and reliably meet a legal compliance limit as low as 280 NTU under all conditions, 

the violation of which can impose heavy civil, and possibly even criminal punishment.

We have demonstrated on numerous construction sites that a fully functioning PTS will drop 

turbidity by 1-2 orders of magnitude or more. My interpretation of the ELG is that there are 

exemptions, such as larger storm events with >2 year recurrence. 

In comments to the proposal, Dr. McLaughlin joined with NRDC in endorsing a 200 NTU 

compliance limit for passive BMP controls, but he seemed to forget that in many of his papers, 

the data was already highly edited to eliminate a very significant portion of the rain events tested, 

and only the most favorable results were reported. The papers explained that they eliminated 

reporting from rain events where the PTS BMPs failed because it was found not to have 

remained intact at the end of the rain event. But this does not adequately explain what caused the 

PTS to fail. If they were properly installed, but actually failed during the rain event because they 

were overwhelmed by the specific rain and site conditions, then the failure is an accurate 

portrayal of problems inherent with setting up PTS BMPs, and these data results should have 

been included if the data is to be used to evaluate a compliance limit. 

This is an unfortunate misinterpretation of the studies. Data were never “eliminated,” but either 

the sampler malfunctioned or there was insufficient flow to obtain a sample. This is clearly 

indicated in the table footnotes. The “failures” were not of the PTS but of the samplers.

In fact, records in the docket show that during rain events at the sites NC-Rd1 and NC-Rd2, 

McLaughlin and his associates, who were operating the systems, were not even at the site during 

the rain events, but were three hours drive away (Docket ID EPA-HQ-OW-2008-0465-2057). 

Not only were they not available to conduct real time interventions (which are not always 

practical to perform safely with true PTS systems), but they also did not have firsthand
knowledge as to what occurred at the site to cause PTS failure. 

The reviewers do not seem to understand that the point of a PTS is that they are passive – no 

intervention is supposed to be needed! While we were not often at the sites during storm events 

(although we were for some), we visited the sites to collect samples soon after storm events and 

took pictures and noted what was occurring on the sites.

The three PTSs used by EPA

already contained the following self-edits to the data initiated by the authors:

 For the NCR.1 site, the McLaughlin paper (Exhibit 10) eliminated five of the twentyseven

rain events (18.5% of the original data) for which the Fiber Check Dam/PAM

BMP was initially evaluated.

SECTIONSEVEN

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 For the NCR.2 site McLaughlin paper (Exhibit 10) eliminated two of the nine rain events

(22% of the original data) for which the Fiber Check Dam/PAM BMP was initially

evaluated.

As noted above, this is apparently a misinterpretation of the tables. No data eres eliminated, but 

automatic sampler issues occasionally prevented samples from being collected. Anyone with 

experience with automatic samplers knows that they can be unreliable for a variety of reasons, 

but when they are working they can provide excellent characterization of storm events.

 For the NC-Road study, the PTS maintenance was supposed to be handled by site

personnel, and not by McLaughlin or his associates. This involved polymer blocs

upstream from a settling basin. The data was reported from 7 sites where the PTS was

confirmed to be intact and functioning. However, the data from nine rain events, or

56% of the original data, was not reported. If the rain event or normal construction site

activity (that did not directly compromise the PTS BMP integrity) was responsible for

the failures, then this data should be included. 

The table (Table 3) refers to a single site with seven storm events, not seven sites. The primary 

treatment was with wattles with PAM applied to them. Installation and maintenance was 

handled by NCSU; site personnel were merely asked not to compromise the PTS or to notify us if 

they did, which they did not. The data not reported were from rain events in which the PTS was 

compromised by the contractor, not by runoff. Some examples include cutting a new diversion 

and inlet into the basin and installing a storm drain pipe that emptied into the basin, all without 

any soil stabilization or erosion protection. It does not make sense to include that data when 

evaluating PTS. 

Also, from Exhibit 10, this site is described as being an LID site involving three buildings, and 

the paper suggests that the LID requirements came into conflict with the PTS. This is at odds 

with EPA claims in the final TDD that this was another North Carolina Road project.

It is very doubtful that Dr. McLaughlin originally wrote these papers with the intent of

establishing a numeric compliance limit for an EPA rule. This means there may be reasons for
rejecting certain data points when experimentally evaluating BMP effectiveness, but these

reasons may no longer be appropriate for rejecting data that is being used to calculate a

statistically supportable NEL. These PTS systems cannot usually be adjusted during actual rain

events, even if operators are present to do so. Therefore, if these PTS “failures” occurred during

the rain event and are caused by normal circumstances during that event, they should have been

included in the calculation of the NEL.

There also are a number of other inconsistencies in data that was not collected at sites NCR.1 and

NCR.2. For example, at site NCR.2 on 9/05/06, no measurement was taken (by the automated

sampling device) due to “low flow” (in the table in the paper there was an entry of “LF” instead

of a value, which keys out to meaning “low flow”). However, this was on a day when there was

a 58 mm (2.28 inch) rain event. It would seem impossible to have low flow for such a large rain

event. Low Flow was again indicated for the 24 mm (0.94 inch) rain event on 7/21/06. In fact,

there seems to have been a problem collecting data for all of the large rain events significantly

above 1.5 inches. Most of the time, these are simply explained as “malfunction occurred”. What

kind of malfunction? One would expect these larger rain events to generate the highest turbidity,

and these would also be the most likely events to overwhelm a PTS such as a fiber check dam. If

that is what occurred, this is not a malfunction; it is merely representative as to how BMPs such

as Fiber Check Dams/with PAM function in the face of large rain events.

The reviewers apparently did not read the footnotes in this paper. The Low Flow indications for 

NCR.2 (site 2 in the paper) cited above had a footnote that stated we were not certain that that 

was what occurred, but either way no samples were obtained. Again, no data were eliminated, 

but the automatic samplers did not collect samples. The three remaining LF events ranged from 

7-17 mm (not “>1.5”, or 38 mm) and LF was highly likely because the wattle/PAM ditches had 

better vegetation and ground coverage. As indicated previously, a “malfunction” indication was 

for the sampler, not the PTS.

As can be seen, much data was already eliminated from consideration for these sites. However,

EPA also intervened to limit the data still further, and in one egregious case completely altered

the statistics for site NCR.2.

NAHB recognizes that some of the EPA deletions were necessary, as in the following

circumstances:

 For NCR.1 EPA deleted the 335 NTU result from 7/25/06 because some telephone crews

had dislodged some of the fiber check dams (see page 6-8 of the TDD).

SECTIONSEVEN

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 For both the NCR.1 and NCR.2 sites, there were rain events measured that took place

after paving was finished and final vegetative stabilization of the site was underway.

EPA deleted values measured after paving was finished.

However, EPA had no technical basis for deleting the following points from NCR.2.

 In the docket, EPA sent an email concerning two daily average turbidity data points from

NCR.2, one was at 533 NTU and one was 261 NTU, stating that these were “higher than

the rest of the data”. These were two of only seven daily average points for the site. EPA

somehow felt that nearly 30% of all the data was somehow due to “outliers”. Dr.

McLaughlin responded by email that (Docket ID: EPA-HQ-OW-2008-0465-2057) “these
spikes were likely due to site activities, but we don’t have anything more specific.” And

“The sites were about three hours away so we couldn’t keep track of all the activities.”

However, these excursions are precisely the type one would expect if a rain event falls

outside the narrow range where the PTS is most effective. Even the best PTS operators

cannot always anticipate site conditions and rain events ahead of time, and unlike the

ATS systems we have discussed, operators cannot intervene during the actual rain event.

There are many appropriate site activities that are not a cause for eliminating these data

points. EPA had no reason to delete these points from the data, especially since there

were so few points. As it turns out, the 261 NTU data point was measured well after

paving was complete, which means that this turbidity was somehow present after the site

was stabilized. This point would have been deleted anyway due to being collected after

paving (see above bullet for explanation). However, EPA did delete the 533 NTU daily

average data point. This rain event was an average of two samples, with the higher

sample being 553 NTU. 

This means the other sample was 513 NTU, so the turbidity was apparently constant over the 

rain event, and the measurement was no anomaly. The elimination of this data point is a major 

change, since the 533 NTU day was one of only four days—25% of the original data that was 

left—where measurements were taken prior to the road being paved at site NCR.2. The 

arithmetic average of the three other data points without this 533 NTU was 49.67 NTU. The 

average with this point included jumps to 170 NTU, along with a similar increase to standard 

deviation and variability. EPA had no sound technical justification for eliminating this point 

from the calculation, especially knowing that a similar high NTU happened again at this same 

site after paving had taken place.

Actually, the 553 in this table is Total Suspended Solids, not turbidity. In reviewing photos from 

dates near that storm event, it appears that paving activities were occurring. Part of the paving 

process is laying down a layer of “crush and run” stone, which is a mixture of stone and fines 

which compact into a stable support layer for the pavement. It is likely the turbidity spike was 

due to the fines washed from this layer prior to paving. These fines may not have been as 

responsive to the PAM we used as the local soil.
Attachment 1. Data From a 3rd Site Near NCR.1 and NCR.2

The project which was the subject of one of the “true” PTS studies (McLaughlin et al., 

2009; NCR.1 and NCR.2 in the URS document) had another road construction site 

which was monitored. This data was not included in the publication because there were 

relatively few storm events in which samples were obtained from all experimental 

sections of the road and because the PTS was destroyed during part of the monitoring 

period. However, the data is consistent with the two sites in the publication, with 

turbidity reductions clearly evident when the PTS was intact (Table 1). 

Table 1. Turbidity averages from a third road construction site not included in the cited 

article (Mclaughlin et al., 2009) because of relative lack of data for direct comparison by 

storm event. Empty cells in the table are the result of either sampler malfunction or 

low flow. All aspects of the monitoring were the same as described in the publication.

Date Standard BMPs Wattles Alone Wattles + PAM

2007 Turbidity (NTU), number of samples in parentheses

Background Samples 

(gravel road only)

1/4 1,015 (24)

1/5 1,679 (23

3/1 1,199 (24)

3/16 5,319 (24)

Road Construction 

Starts

4/15 5,744 (24) 5,335 (24)

6/12 7,310 (2) 254 (24)

6/14 7,915 (5)

Wattles Dislodged

6/29 4,771 (1) 10,308 (8)

7/17 3,470 (3)

7/25 3,951 (5)

7/27 5,302 (4) 7,157 (6)

8/26 3,840 (3)

Wattles Reinstalled

9/14 1,613 (15) 5,507 (24) 951 (24)

10/24 667 (8) 36 (9)

Road Paved

12/18 52 (2)

12/23 187 (6)

12/28 521 (11)

1/11 (2008) 171 (1)

1/17 23 (13)
Attachment 2. Preliminary Data from a Current Project

We are currently conducting further studies on another NC DOT highway construction 

project in our area. On two skimmer basins we have installed simple PTS and are 

monitoring at both the inlet and outlet. The site is in mass grading and highly disturbed, 

but the BMPs are installed and maintained quite well. The inlets are 12” corrugated 

pipe with gravel/stone inlet protection, and each basin has a forebay with 12” corrugated 

pipe overflow into the second chamber (Figure 1). Coir netting was placed on the 

gravel as a carrier for granular polyacrylamide (PAM) applied to the netting. We also 

inserted jute netting into the upper end of each pipe, wetted the fabric, and applied PAM 

to that. The storms we have monitored so far have produced flows in the 100-200 

gallons per minute range, well below the design storm flows. However, as you can see 

in Table 2, we have had very good results in reducing turbidity in these basins.

Table 2. Average turbidity in samples at the basin inlet and outlet (skimmer) for two 

basins on an active highway construction site in Wake County, NC. 

Date (2010) Basin 3 IN Basin 3 OUT Basin 4 IN Basin 4 OUT

Rain total in 

parentheses.

Average Turbidity (NTU), number of samples in parentheses.

9/29 (2.1”) 386 (24) 267 (24) 213 (24) 34 (5)

10/14 (0.54”) 5,490 (3) 32 (8) 29,500 (6) 175 (24)

11/4-6 (0.70”) 5,880 (5) 57 (11) 777 (7) Sampler 

Malfunction

11/17 (0.33”) 17,000 (1) 73 (4) 21,600 (16) 124 (18)
 

Figure 1. Photos of the inlet (top) and forbay (bottom) of Basin 3. The rocks with coir 

fabric on them are for inlet protection from heavy sediment, which is settling in the ditch. 

Pipe inlet