Using GIS to Explore Water Quality

Using GIS to Explore Water Quality

Trends in the Middle Rio Grande

Background:

The Middle Rio Grande (MRG) extends from Cochiti Reservoir in north central New Mexico ~ 300 kilometers south to Elephant Butte Reservoir. This section of the Rio Grande is divided into two geomorphically distinct reaches with a constrained high gradient section in the north and an unconstrained, low gradient, sand bottom system in the south (Figure 1). The wide floodplains and moderate topography of the southern section of the MRG have led to significant urban and agricultural development. The urban development is concentrated in the northern section of the unconstrained reach (Figure 2) with the agricultural land centered in the middle of the reach. Water quality in the MRG has important implications for 1) the endangered silvery minnow which is endemic to the Rio Grande (The MRG Endangered Species), 2) the residents of Albuquerque who will soon be using river water as a municipal drinking water source, and 3) the water quality of downstream reservoirs. In September, 2005 I began monthly collection of water samples to quantify the spatial and temporal trends in water quality of the MRG. Samples were collected at 25 sites located downstream of all major tributaries (Figure 3) and were analyzed for nutrients, major anions and cations, and dissolved organic and inorganic carbon.

Objectives:

The objectives of this GIS project were to 1) determine the river distance between sites to allow spatially explicit graphing of the data, 2) use the spatially explicit graphing to calculate nutrient uptake lengths, and 3) explore the geology/soil properties of selected tributary watersheds to help explain changes in water quality that may be due to subsurface tributary inputs to the river.

Results and Discussion:

1) The distance between sites was calculated and used to graph the data below (Calculating site distances). As the MRG moves from the upper, geomorphically constrained reach where urban development is limited into the unconstrained, developed area, concentrations of all nutrients and conservative solutes increase dramatically (See figures below). This increase can be attributed to waste water treatment plant point source inputs.

Abq WWTP Input

Abq WWTP Input

2) The slow decline in nutrient concentrations with downstream distance from the waste water inputs seen in the figures above can be attributed to nutrient uptake by either biotic or abiotic mechanisms. Nutrient uptake rates can be calculated from this decline and can be used to compare uptake between months (Figure 4). These data show a clear relationship between river discharge and uptake rates and lengths (See table below).

Month	Q at San Marcial (cfs)	Uptake Rate NO₃ (mg/km)	Uptake Length NO₃(km)	Uptake Rate PO₄ (mg/km)	Uptake Length PO₄(km)
October	57	0.0124	81	0.0106	94
November	410	0.0066	152	0.0043	233
December	469	0.0035	286	0.0025	400
January	640	NA	NA	0.0071	141
February	475	0.0038	263	0.0089	112

3) Longitudinal conductivity data show a significant increase in the urbanized section of the MRG most likely due to waste water treatment plant inputs (See figure below). Levels remain elevated until the confluence of the Rio Grande with the Rio Puerco and Rio Salado where there is a significant increase in conductivity even when there is little or no flow in either tributary (Rio Puerco Q = 0.14 cfs in February). This increase is most likely due to subsurface inputs from these two watersheds which flow through ion rich geologic formations and soil deposits. An analysis of the geology of the Rio Puerco basin shows that the seven most abundant geologic components, which make up 65 % of the water shed geology, are sedimentary (Geologic analysis). Soft sedimentary rocks are more likely to contribute ions to surface and groundwater than harder igneous rock. A qualitative exploration of the STATSGO soil properties for the Rio Puerco shows that some of the soils adjacent to the stream outlet are Torripsamments and Torriothents which have high salinity and calcium carbonate values and may add significant ion contributions to subsurface flow.

Component Area	Component Name	Component Type	% of Total Area
0.693	San Jose	Sedimentary	17.4
0.643	Menefee	Mudstone	16.1
0.364	Nacimiento	Sedimentary	9.1
0.27	Crevasse Canyon	Sandstone	6.8
0.246	Kirtland	Sedimentary	6.2
0.239	Alluvium	Sedimentary	6.0
0.142	Chinle Group	Sedimentary	3.6

Conclusions:

Within the urbanized portion of the MRG waste water treatment plants are a significant point source for nutrients and conservative solutes. As nutrients are transported away from the urban area, concentrations decline probably due to microbial processing. The ability of the river to process waste water inputs appears to be highly dependent on river discharge. Discharge levels affect both the interaction of the transported solutes with the benthos where biotic uptake occurs, as well as suspended solid transport which may affect abiotic adsorption of phosphates. Natural subsurface inputs from geologically distinct tributaries appear to contribute significantly to conservative ion transport as measured by specific conductivity.