Comparison of rhodamine WT and bromide in the determination of hydraulic characteristics of constructed wetlands

doi:10.1016/S0925-8574(03)00005-3

Ecological Engineering

Volume 20, Issue 1, March 2003, Pages 75-88

https://doi.org/10.1016/S0925-8574(03)00005-3 Get rights and content

Abstract

Hydraulic tracer tests were performed in the Prado Wetlands, Riverside County, California, USA. The goals of the tests were (1) to evaluate the suitability of rhodamine WT (RWT) as a tracer for wetlands studies, and (2) to determine the residence time distribution of the wetlands. The performance of RWT was evaluated by comparing the breakthrough curve (BTC) of RWT to that of bromide in a pilot-scale test. The BTCs of RWT and bromide indicated equal results. After the pilot test, a full-scale test was conducted by releasing a RWT pulse at the wetlands inlet and monitoring for RWT arrival near the wetlands outlet. The BTC indicated 10 and 90% (of the total mass recovered) breakthrough times of 25 and 112 h, respectively, but these must be considered approximations because only 29% of the injected RWT mass was recovered. Laboratory experiments suggest irreversible sorption to be the principal loss mechanism of RWT during transport through the wetlands. RWT is a suitable tracer in wetlands that are relatively small (less than 1 week residence time) and deep (at least 0.6 m) with limited sediment contact, but RWT yields only approximate results for the extended wetlands system.

Introduction

Natural and engineered wetlands are emerging as important treatment systems for wastewater and agricultural run-off, especially for the removal of nitrate (Bachand and Horne, 2000, Reilly et al., 2000), trace metals (Debusk et al., 1996, Kadlec and Knight, 1996, Webb et al., 1998), pesticides (Schulz and Peall, 2001), and industrial solvents (O'Loughlin and Burris, 1999). The factors that govern contaminant removal in wetlands are poorly understood, but uptake by plants, sorption by sediments, microbial degradation and precipitation have been implicated. Greater reliance on wetlands treatment and optimization of existing wetlands operations require a better understanding of the hydraulic and geochemical factors that govern contaminant behavior. A precondition for studying wetlands hydraulics is the availability of robust tracer methods for assessing the residence time distribution and mixing conditions.

The goals of this study were (1) to evaluate the use of Rhodamine WT (RWT) as a tracer for wetlands studies, and (2) to hydraulically characterize the Prado Wetlands (Riverside County, California) using RWT. The Orange County Water District (OCWD) diverts a significant (up to 50%) fraction of the Santa Ana River (SAR) to the 130-hectare Prado Wetlands. Flow through the wetlands improves the quality of the SAR in that 200 tonnes of nitrate is removed per year (OCWD, 2001). SAR flow is the primary source for augmenting the Orange County groundwater basin (OCWD, 1995), and maintaining the basin's water quality is one of the primary concerns of the OCWD.

The suitability of RWT was evaluated in a pilot test by comparing its performance to that of bromide. Although bromide is a conservative tracer (Netter and Behrens, 1992, Tanner et al., 1998), its application for wetlands may be problematic because (1) it is a potential precursor for disinfection by-products in drinking water supplies, and (2) large additions are necessary if background concentrations are relatively high (e.g. ca. 0.2 mg/l in the SAR). In addition, on-site analysis of bromide is difficult. By contrast, RWT is accepted by regulators and is easily detectable with a portable fluorescence detector at concentrations as low as 0.01 μg/l (Wilson, 1968, Smart and Laidlaw, 1977, Kilpatrick and Wilson, 1982). The interference of background fluorescence is generally insignificant. RWT has previously been used in a mountain stream (Bencala et al., 1983) and for groundwater tracing (Smart and Smith, 1976, Ptak and Schmid, 1996, Pang et al., 1998).

RWT is a large aromatic molecule (Fig. 1) with a maximum fluorescence emission wavelength of 580 nm (Smart and Laidlaw, 1977). The drawback of RWT as a tracer is that it behaves non-conservatively in certain situations, either by sorbing to sediments (Smart and Laidlaw, 1977, Bencala et al., 1983, Trudgill, 1987, Shiau et al., 1993, Everts and Kanwar, 1994), or by degrading photochemically (Smart and Laidlaw, 1977, Tai and Rathbun, 1988) or biologically (Smart and Laidlaw, 1977). Sorption to vegetation was found to be relatively unimportant (Turner et al., 1991). Tracer studies with RWT that last longer than a week may need to consider photochemical decay (Smart and Laidlaw, 1977). Biological decay is believed to be negligible in surface water where microbial populations are relatively low (Smart and Laidlaw, 1977). Sorptive loss can be significant but is difficult to evaluate in field situations. Except for a study in a tidal marsh (Tarrell, 1997), RWT has not been evaluated as a tracer for wetlands studies where sediment contact and sorption by plant material may play a significant role.

As part of this study, Prado Wetlands sediment was evaluated to assess the capacity of the sediments to retard RWT transport and/or to act as an irreversible sink for RWT. Experimental objectives of the study were to determine the sorption isotherm and to evaluate the reversibility of sorption. In natural environments, RWT sorption has been shown to depend on initial dye concentrations, sediment type, and organic matter content (Smart and Laidlaw, 1977, Trudgill, 1987, Everts and Kanwar, 1994). Reported K_oc values (defined as the distribution coefficient, K_d, normalized by the organic carbon fraction, f_oc) are often on the order of 1000–4000 ml/g (Sabatini, 1989, Sabatini and Austin, 1991, Trudgill, 1987), though some studies report apparent values over 10 000 ml/g (Trudgill et al., 1983, Shiau et al., 1993). Prediction of RWT sorption based on empirical relationships using the octanol–water partitioning coefficient, K_ow, and the sediment organic fraction, f_oc, often underpredict the actual observed RWT sorption (Sabatini and Austin, 1991). It has been speculated that, under certain conditions, the functional groups of the RWT molecule (see Fig. 1) may dominate over organic partitioning as a sorption mechanism (Kasnavia et al., 1999).

Section snippets

Description of study area

The Prado Wetlands (Fig. 2) cover 130 ha of open surface water and were originally built for duck hunting. In 1992, the OCWD began studying the efficiency of the Prado Wetlands for nitrate removal (OCWD, 1995), and in 1998, the wetlands were redesigned to improve their performance for that function.

The Prado Wetlands consist of four subsections that each consists of inter-connected cells (total of 46 cells). The four subsections are termed East Cells, North Cells, South Cells, and West Cells,

Pilot test

Fig. 3 depicts the flow rate during the pilot test, and Table 2 summarizes the general conditions and residence time distributions of the two tracer tests. During the pilot test (151 h), the flow rate varied between 345 and 79 l/s. For the first 60 h, the flow rate was relatively constant (in the range of 310 l/s), and then it steadily decreased to 85 l/s at 100 h before it increased again to 140 l/s. The time-averaged flow rate during the pilot test was approximately 210 l/s, resulting in a

Conclusions

1
In the pilot test, which covered two adjacent 0.76 m deep wetland cells, mass balances for RWT and bromide were 59 and 85%, respectively, and the breakthrough responses for RWT and bromide were similar. The residence time distributions predicted by RWT and bromide, defined as the 10 and 90% breakthrough points, were essentially equal for both tracers. The apparent mean residence time defined by a first moment analysis is 55 h for bromide and 53 h for RWT. The difference of 2 h (<4%) is within

Acknowledgements

This research was funded by Orange County Water District (OCWD) located in Fountain Valley, California. We are grateful for their support and assistance in field wetland research. Specially, we thank the staff of the OCWD, especially Gerry Bischof, Katherine O'Connor, Patrick Tennant, and Greg Woodside. We also thank Tara Schraga and Carry Lopez from USGS located in Menlo Park, California, for assisting and letting us use their fluorometer for sorption experiments. Any opinions, findings,

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Comparison of rhodamine WT and bromide in the determination of hydraulic characteristics of constructed wetlands

Abstract

Introduction

Section snippets

Description of study area

Pilot test

Conclusions

Acknowledgements

J. Hydrol.

Chem. Eng. Sci.

J. Hydrol.

J. Hydrol.

Chemosphere

J. Hydrol.

Thermal mediation by littoral wetlands and impact on lake intrusion depth

Water Resour. Res.

Denitrification in constructed free-water surface wetlands: I. Very high nitrate removal rates in a macrocosm study

Ecol. Eng.

Rhodamine WT dye losses in a mountain stream environment

Water Resour. Bull.

Retention and compartmentalization of lead and cadmium in wetland microcosms

Water Res.

Treatment Wetlands

Fluorecent dye and media properties affecting sorption and tracer selection

Ground Water

Techniques of Water-Resources Investigations of the United States Geological Survey

Effectiveness of various treatments in retarding microbial activity in sediment trap material and their effects on the collection of swimmers

Limnol. Oceanogr.