Distribution of Nitrate Contamination in the Salinas Valley

The Salinas Valley is one of the most agriculturally productive regions in the United States. As a result of extensive agricultural activity, nitrate contamination issues plague groundwater resources throughout the region.

The maximum contaminant limit (MCL) for nitrate in drinking water is set at 45 mg/L (as Nitrate). Above this level, nitrate is harmful to human health, causing blue baby syndrome in infants and stomach cancer in adults.

The Salinas Valley relies almost completely on groundwater for both irrigation and drinking water supply. Therefore, it is important to monitor the distribution of nitrate contamination in order to protect groundwater supply and ensure clean drinking water.

The objective of this research is to assess the distribution of nitrate contamination within the Salinas Valley and identify the urban areas most at risk to nitrate contamination within the Salinas Valley.

URBAN AREAS

There are eight urban areas identified within the Salinas Valley. The largest urban areas are in the northern portion of the basin. Smaller urban areas are present in the central portion of the basin. A single urban area is identified in the southern portion of the basin.

https://baboyle.carto.com/builder/40c78568-3073-11e7-b2e1-0e233c30368f/embed
SELECT 
urban_areas.cartodb_id,
urban_areas.name10,
urban_areas.the_geom_webmercator
FROM 
urban_areas,
salinasvalleygwbasin
WHERE    
ST_Intersects(urban_areas.the_geom_webmercator,
salinasvalleygwbasin.the_geom_webmercator)
GROUP BY 
urban_areas.cartodb_id,
urban_areas.name10,
urban_areas.the_geom_webmercator

Nitrate MCL Exceedances Through Time

MCL exceedances have been observed throughout the basin. Locations of MCL exceedances, in general, have not varied widely over the last five years. The most exceedances are observed in the northeastern portion of the basin, with exceedances also observed in the central and southern portions of the basin.

Interestingly, the year 2016 appears to have few MCL exceedances. This is likely due to incomplete data: complete water quality data has likely not been released for public use at the time data was downloaded.

https://baboyle.carto.com/builder/bb2607c6-3398-11e7-8d88-0e3ff518bd15/embed
SELECT 
cartodb_id,
the_geom_webmercator,
well_id,
COUNT(result),
EXTRACT(Year from date_) as Year
FROM 
mcl_exceedances
GROUP BY 
cartodb_id,
the_geom_webmercator,
well_id,
Year

Count of MCL Exceedances By Subbasin in the Last Five Years

In the last five years, the most MCL exceedances were observed in the northern and northeastern portion of the basin. The East Side Aquifer subbasin has had the most MCL exceedances in the past five years, followed by the 180/400-Foot Aquifer subbasin and Langley Areaa subbasins, respectively.

https://baboyle.carto.com/builder/eaae41d2-38e5-11e7-83f9-0e233c30368f/embed
SELECT cartodb_id, the_geom_webmercator, subbasin_n, count(result)
FROM
(
SELECT
salinasvalleygwbasin.cartodb_id,
salinasvalleygwbasin.the_geom_webmercator,
salinasvalleygwbasin.subbasin_n,
mcl_exceedances.result
 FROM
salinasvalleygwbasin
 INNER JOIN
mcl_exceedances
 ON
ST_Contains (salinasvalleygwbasin.the_geom_webmercator,
mcl_exceedances.the_geom_webmercator)
) as subview
GROUP BY cartodb_id, the_geom_webmercator, subbasin_n

Count of MCL Exceedances Within Five Miles of Urban Areas in the Last Five Years

Contamination of drinking water is a concern within the Salinas Valley Groundwater Basin. In order to determine the urban areas most at risk of nitrate contamination, the number of MCL exceedances within five miles of each urban center for the last five years were summed. The urban area most at risk to nitrate contamination is Salinas, followed by Gonzalez and Soledad, respectively.

https://baboyle.carto.com/builder/3512a3aa-3380-11e7-a1d5-0ee66e2c9693/embed
SELECT cartodb_id, the_geom_webmercator, name10, count(result)
FROM
 (
 SELECT 
urban_areas_sv.cartodb_id,
urban_areas_sv.the_geom_webmercator,
urban_areas_sv.name10,
mcl_exceedances.result
 FROM 
urban_areas_sv
 INNER JOIN 
mcl_exceedances
 ON 
ST_DWithin (urban_areas_sv.the_geom_webmercator,
mcl_exceedances.the_geom_webmercator, 8046.72)
 ) as subview
GROUP BY cartodb_id, the_geom_webmercator, name10

Data Sources

Water quality data for this assessment was obtained from CA Groundwater Ambient Monitoring and Assessment (GAMA), CA Geotracker, and USGS National Water Information System.

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