Kawa Stream TMDL Project

Water quality characteristics of Kawa Estuary

REPORT INTRODUCTION Methods Results
Introduction

The estuary of Kawa Stream is approximately 3500 feet (1070 m) long extending from a dense growth of hau some 15 m (50 ft) downstream of the concrete structure in the channel near the upper golf cart bridge on Bayview Golf Course to an outlet through mangroves at the seaward wall of a former fishpond (Waikalua) in southern Kane`ohe Bay. The channel is man-made and partly lined with red mangrove (Rhizophora mangle).

The discussion presented here considers various facts know about the estuary, its physical structure, and water quality based upon measurements made since 1991, but most particularly those made recently by the Department of Health (DOH). The purpose of this effort is to allow an estimate of the impacts of Kawa Stream on water quality in south Kane`ohe Bay as part of a TMDL study for Kawa Stream. The analysis is aided by the fact that half of the Kawa Stream stations regularly monitored by DOH are located along the estuarine reach of the stream. Land use surrounding a substantial portion of this reach is golf course.

Methods

The water in Kawa estuary is not just brackish, but dynamically mixed: stream flow and ocean tide carry out a physical process which combines the two water bodies. This process is progressive from the upper end of the estuary under ordinary stream flow conditions. Under high flow conditions (freshets), the mixing zone may be pushed out of the physical channel into south Kane`ohe Bay.

A calculated dilution factor based upon the salinity at each station is used to predict concentration expected from this mixing process. If we assume that salinity in a brackish sample represents a mixture of stream water with no salt (0 ) and sea water (35 salt), then a dilution factor for any given salinity is arrived at from the equation:

Dilution Factor = -0.02857 x salinity + 1

The lower-most fresh water WQ monitoring location is Station 5, some 50-60 ft upstream of the upper end of the estuary. The expected concentration for each monitored nutrient parameter downstream of Station 5 can be calculated by multiplying the mean concentration at Station 005 by the Dilution Factor calculated from the mean salinity for each estuary station. Then a correction is made for levels of each substance in the "diluting water" of Kane`ohe Bay. Mean values for nutrient data collected in this area by Coles and Ruddy (1995) are used as background concentrations for these corrections (Table 1). The background concentration for each substance is first multiplied by 1/Dilution Factor (i.e., divided by the dilution factor) and this product is added to the station expected concentration derived above.

Table 1. Baseline nutrient concentrations (ug/l) for
south Kane`ohe Bay near Kawa Stream (from Coles & Ruddy, 1995)

NO3 + NO2NH4TNTP
MEAN CONCENTRATION
3
713321

Results

The distribution of all nutrient species (except ammonia) in the estuarine portion of Kawa Stream generally follow the predicted pattern based solely on dilution with Kane`ohe Bay water (Figures 1 to 4). Nitrate and total phosphorus levels decrease somewhat more rapidly than is predicted. For total phosphorus this could represent loss due to settling out of particulate matter as water moves slowly out of the the estuary. For nitrate, uptake by vegetation and/or phytoplankton may be implied. These explanations could perhaps be better documented by measuring dissolved organic nitrogen (DON) and phosphorus (DOP) at the estuarine stations.

The increase in ammonia concentration at Stations 2 - 4 suggests that nitrification is active in the bottom sediments along this part of the estuary. This interpretation presumes that there is a significant source sediments containing organic nitrogen here, something not yet verified by observation. It is also possible that some extraneous input of ammonia is occurring as runoff or seepage into the estuary. If the excess ammonia (i.e., actual less estimated) is subtracted from total nitrogen at Stations 2 through 4, then the observed distribution of total nitrogen more closely approximates estimated total nitrogen (see Figure 3).


Figure 1. Change in nitrate levels (actual and estimated) as a function of dilution in the estuarine reach (ELR) of Kawa Stream (based on DOH data).

Figure 2. Change in ammonia levels (actual and estimated) as a function of dilution in the estuarine reach (ELR) of Kawa Stream (based on DOH data).

Figure 3. Change in Total N levels (actual and estimated) as a function of dilution in the estuarine reach (ELR) of Kawa Stream (based on DOH data).

Figure 4. Change in Total P levels (actual and estimated) as a function of dilution in the estuarine reach (ELR) of Kawa Stream (based on DOH data).
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