PAM Peer Review Panel

The PAM Peer Review Panel (PRP) was established to provide guidance to the experiments conducted by the Research Consortium and to provide feedback on a toxicological risk characterization document.

The PRP members included:

Summary of Findings

As a result of a literature review and after an evaluation of the data obtained from application in test canals, the following potential risks associated with the use of LA-PAM were identified: (1) ecological and human health impacts associated with the environmental release of residual AMD and (2) the physiological impacts on benthic organisms of LA-PAM release into receiving streams.

Acrylamide is a known cumulative neurotoxin and a suspected human carcinogen. Based on an AMD concentration of 0.05 percent in the polymer and LA-PAM at a typical use level of 1 ppm (1 mg/L), the AMD drinking water standard has been set at 0.5 ppb (0.5 µg/L). Though application of LA-PAM in canals is expected to be used in similar or slightly higher concentrations than the drinking water standards, LA PAM applications will occur only one to two times per year at a rate of no more than 10 pounds/canal acre (lbs/ac) per application. AMD would be released only during the application period (in most drinking water treatment applications, PAM is added continuously).

The largest expected concentration of AMD in water delivery systems occurs when dry LA-PAM is added to dry soil in the canal, followed by the flushing of the canal with irrigation water. Under this application method, LA-PAM concentrations in canal water might be higher than allowed under the safe drinking water standards, but only for brief periods of time (on the order of hours), if at all. This pulse is transient and is expected to dilute as the water is transported through the canal systems. Given the large size of the LA-PAM molecule when hydrated, LA-PAM is not expected to migrate through soil material or into groundwater aquifers. When LA-PAM is applied to a flowing canal as specified by the application protocols currently being developed by the U.S. Bureau of Reclamation (USBR) and the Research Consortium (RC), the maximum AMD concentration is expected to be below the 0.5 µg/L drinking water standard. Using these protocols during controlled field experiments (i.e., dry LA-PAM was applied to a water-filled canal at an approximate rate of 10 lbs/ac), AMD concentrations measured in canal water have, to date, remained below 0.50 µg/L for all but two samples, out of 35 samples analyzed to date. Most samples (23 of 35) had concentrations less than 0.25 µg/L. Though this “dry on flowing” field application does not encompass all possible application methods, it is commonly used by stakeholders. Nonetheless, only limited information on AMD concentrations is presently available, so only a risk characterization can be provided at present.

The risk characterization contained in this report focused on two potential pathways for AMD exposure as a result of using LA-PAM: ingestion and inhalation. Ingestion of AMD was considered to be the more probable exposure pathway, and would most likely occur through the consumption of (1) canal water immediately after LA-PAM treatment and/or (2) groundwater impacted by AMD. AMD has a very low volatilization potential when dissolved in water and thus does not pose a health risk by way of the inhalation route. Dermal exposure of LA-PAM is not considered given the availability and the recommended use of personal protective equipment by workers applying LA-PAM. Although this document does not address occupational exposure to AMD as a result of applying the LA-PAM per se, it is recommended that personnel use protective equipment (e.g., dust masks) to reduce exposure to airborne particulate LA-PAM containing AMD during the application process.

As part of the risk characterization, AMD concentrations in treated water were calculated based on realistic canal geometries, flow characteristics, and conservative assumptions about LA-PAM hydration and AMD release rates. In addition, concentrations of acrylamide were monitored in treated canals. AMD concentrations in treated waters were compared to current U.S. EPA drinking water standards, and to concentrations derived from the lowest doses that caused adverse effects in animal studies of AMD toxicity, as well as to concentrations equivalent to the highest doses that caused no adverse effects in these studies. The effects of AMD on laboratory animals were extrapolated to humans by applying an uncertainty factor (UF) to the animal doses to account for uncertainty inherent in assuming that sensitive human receptors would respond in the same fashion as animals, and for extrapolating less than lifetime exposures to lifetime exposures. The uncertainty factor of 1,000 used by the U.S. EPA for the chronic acrylamide oral reference dose (RfD) was applied for this exercise (U.S. EPA, 1988). Analytical results of samples collected in operational canals during controlled field experiments indicate that observed concentrations were 50 percent below drinking water standards, approximately three orders-of-magnitude below the UF-adjusted lowest daily doses that caused reproductive impacts in laboratory animals, and about one-fourth of the drinking water concentration associated with a one-in-ten thousand lifetime risk for cancer if consumed over a lifetime according to the U.S. EPA.

Based on the knowledge gained from field and laboratory experimentation, and assuming that LA-PAM is applied in accordance with the draft application protocols proposed by the Research Consortium, the following conclusions were made:

1. The concentration of LA-PAM could, for brief periods of time during field application, exceed the Safe Drinking Water Act treatment technology limitation of 1 mg/L polymer. Concentrations of AMD are likely to remain below the drinking water treatment technology standard 0.5 µg/L monomer from a certified polymer. To date, of 35 water samples collected and analyzed during field experiments, 18 were below 0.1 µg/L of AMD, 14 were below 0.5 µg/L, and two were above 0.5 µg/L and less than 0.7 µg/L. Studies being conducted in the laboratory indicate that incipient AMD in the LA-PAM formulation degrades relatively rapidly in the environment, and does not accumulate/bioaccumulate, and no AMD is formed in significant amounts from breakdown of the polymer in the environment.

2. Elevated concentrations of LA-PAM and AMD are expected in surface water samples immediately after LA-PAM addition to a canal, and in close proximity to the locus of addition. These elevated concentrations will be transient and depend on the application time (<1 to 12 hours) in a specific reach of a canal being treated. It is expected that this condition will occur only one to four times per year.

3. The highest concentrations of AMD expected (based on limited field data) are nearly two orders of magnitude below the No-observed-adverse-effect–level (NOAEL) for human receptor surrogates. Consequently, little effect is expected for AMD, from an ecotoxicological perspective.

4. The highest LA-PAM concentrations expected will be in canal sediments. Effects of LA-PAM usage on benthic organisms associated with LA-PAM usage are still being investigated through field experiments, but preliminary results show elevated benthic organism drift rates for about 11 hours after PAM addition; other impacts to benthic communities are still being examined. Until these research findings are completed, LA-PAM use should be avoided near biologically sensitive canal systems.

5. Depending on the assumptions used in calculations, and using the results collected in full-scale canal tests, the predicted concentrations of AMD in canal water will –remain close to or below the drinking water standard (0.5 µg/L). Furthermore, field samples collected to date have contained AMD at concentrations consistently lower than the predicted concentrations. Human exposure of AMD from ingestion of canal water is low, and exposure from potential groundwater contamination would be progressively less. Even if AMD could reach groundwater systems from the transient pulse of AMD in the canal, concentrations of AMD would be below levels observed in canal water, and would be further diluted in groundwater as it moves.

6. Additional information on the environmental fate of AMD and LA-PAM is necessary for a comprehensive risk assessment of the use of LA-PAM for seepage control, especially as it relates to degradation pathways for AMD in the ambient and groundwater environment.

7. Though the findings by Manson et al. (2005) (see Sections 2.2.2.2 and 2.2.2.3 in the report) have significant bearing on the use of LA-PAM in canals, the exposure analysis conducted in this report indicates that acute (short-term) AMD concentrations in canal waters will be between one and four orders of magnitude below the chronic (long-term) levels needed to impact human health.

The U.S.EPA is presently preparing an updated health risk assessment for acrylamide as part of the Agency Integrated Risk Information System (IRIS) program. The existing IRIS assessment was completed in 1988. Since that time a number of new studies of the cancer and noncancer health risks associated with exposure to AMD have been published. These studies will be included in the new IRIS Toxicological Review for Acrylamide. Agency estimates that the updated assessment will be peer reviewed in December of 2007 (http://cfpub.epa.gov/iristrac/index.cfm); it will be made publicly available prior to the peer review.

Download the full Risk Characterization: Using Linear Anionic Polyacrylamide (LA-PAM) to Reduce Water Seepage from Unlined Water Delivery Canal Systems document

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