Explanation for National Analysis of Nutrient Concentrations in Streams and Rivers

 

Provided for Use in Summary Reports of the 1994 NAWQA Study Units

Excel spreadsheet:  “sw_nuts_summ_727.xls”

July 27, 2000

By David K. Mueller

(updates from Feb. 14, 2000 in blue text)

(updates from Apr. 10, 2000: Deleted NH4, NO3 and TN values for SCTX 08198000 and TP values for UTEN 03465500 because of poor model fit)

 

Flow-weighted mean concentrations for 5 nutrient species are presented for 372 fixed sites in the 1991 and 1994 NAWQA study units.  The nutrient species are ammonia (NH4 – includes dissolved ammonium plus unionized ammonia), nitrite plus nitrate (NO3), total nitrogen (TN), orthophosphate (PO4), and total phosphorus (TP).  A regression model was calibrated to estimate the daily load of each constituent at each site.  The calibration data included periodic sample concentrations and daily streamflow measurements.  The models were used to estimate daily loads of each species based on daily streamflow for a period of 1 or 2 water years.  The daily loads and daily streamflow volumes were summed for the entire period, and the ratio of the sums was used to calculate the flow-weighted mean concentration for each species.

Data, model, and results

The concentrations of TN used in model calibration were calculated from measured concentrations of total Kjeldahl nitrogen (TKN - ammonia plus organic N) and NO3.  If NO3 was less than detection (0.05 mg/L), TN was assumed to be the concentration of TKN.  If TKN was less than detection (generally 0.2 mg/L), TN was assumed to be <0.2 or the concentration of NO3 if this was >= 0.2.

 

The model considers concentrations that are less than detection, but requires a minimum of 7 values greater than detection for calibration.  At many sites, one of more species failed to meet this criterion.  In these cases, the flow-weighted mean concentration was estimated to be less than the highest detection limit during the sampling period.  In some other cases, where a model estimate was possible, the flow-weighted mean concentration was less than this detection limit.  For non-biased comparison among sites, these estimates were censored at the highest detection limit for each species, which was considered the minimum reporting level.  Mean-annual concentrations computed less than the laboratory reporting level of for total phosphorus, less than the long-term method detection limit of 0.030 mg/L are shown as negative values in blue text color in the spreadsheet.

 

The flow-weighted mean concentrations were checked by comparing TN to the sum of NH4 plus NO3 and by comparing TP to PO4.  Discrepancies were evaluated and rectified by replacing the concentration that seemed to be erroneous.  In most cases, TN values were replaced by the sum of NH4 and NO3 and TP values were replaced by PO4.  In a few cases, NO3 values were replaced by the difference of TN and NH4.  These replacements were made under the assumption that particulate N or P was negligible at the affected sites.  The replacement concentrations are not necessarily accurate, but are reasonable for use in relative ranking among sites.  All replacements are shown in red in the spreadsheet.

 

In a few cases, the model could not be calibrated or substantial errors were indicated.  In these cases, no estimated concentration was included in further analyses, and the value in the spreadsheet is a blank.

Percentiles

In addition to the estimated flow-weighted mean concentrations, the spreadsheet lists the percentile of concentration for each species at each site. Percentiles for each site indicate the relative ranks of nutrient species concentrations at that site in comparison to all other NAWQA sites where estimates for that species are available.  Summary statistics for each species are shown in the following table.

 

Species

Sites
(total = 372)

Flow-weighted mean concentration (mg/L)

25th P’tile

Median

75th P’tile

Maximum

Mean

NH4

372

0.025

0.046

0.098

8.6

0.12

NO3

368

0.22

0.71

2.0

15.

1.8

TN

371

0.64

1.3

2.9

19.

2.5

PO4

369

0.012

0.033

0.10

2.4

0.089

TP

371

0.049

0.12

0.27

2.7

0.20

 

Ancillary Data (including classification of land use)

The spreadsheet also lists selected ancillary data for each site.  Basin areas, land use percentages, population, and crop group are from national synthesis analyses based on national GIS coverages and study-unit submitted basin boundaries.  Land use classification is generally based on netcodes submitted by the study units.  The classification ‘undeveloped’ is used for reference and forest sites.  The classification ‘mixed’ is used for integrator sites and for some indicator basins with no predominant land use.  In some cases, classifications do not match netcodes.  These changes were made so that classifications were more consistent on a national basis (or in a few cases, to correct errors).  Classifications shown in blue (11 sites) were changed and verified by the study unit.  Classifications shown in red (13 sites) were changed, but not yet verified.  Classifications shown in green (13 sites) were changed from the first version of the summary table, primarily to correspond with the classifications used in the National Summary report, USGS Circular 1225.  The crop group was changed for a few sites with <=0.2 percent agricultural land to indicate ‘zero crop acreage’.