Water quality analysis

Analytical methods used by the laboratory

The current analytical methods used for measurement of samples collected from the Regional Water Monitoring Partnership (RWMP) sites are described below.

pH

pH should be determined in the field, (recording temperature at time of measurement).
The current supplementary laboratory analytical method for the determination of pH is Standard Method No. 4500 (APHA, 2012). The analysis is performed on an autosampler combined with a pH meter. The meter is calibrated daily on 2 pH buffers with a third control buffer analysed as a check. Water washes are included between samples to ensure no cross contamination.

Electrical conductivity (µS/cm at 25°C)

The current analytical method for the determination of electrical conductivity is Standard Method No. 2510B (APHA, 2012). The analysis is performed on an autosampler combined with an electrical conductivity meter. The meter is calibrated daily with 0.001M, 0.01M and 0.1M potassium chloride solutions to ensure adequate linearity. The temperature compensation is also checked daily. The results are reported to the nearest integer from 2 µS/cm at 25°C to 100 µS/cm at 25°C. Above this the results are reported to 2 significant figures.

True colour (Pt-Co units expressed as filtered colour)

The current analytical method for the determination of true colour is Standard Method No. 2120B (APHA, 2012). The analysis is performed after filtering the sample through a 0.45 micron membrane filter. Samples are compared visually against a series of platinum-cobalt (Pt-Co) standards using 100 mL Nessler tubes against a white tile background. Method QC requires a visual check by a second analyst.

Suspended Solids (mg/L)

The current analytical method for the determination of suspended solids is Standard Method No. 2540 D&E (APHA, 2012). The analysis is performed by filtering a measured volume of sample through a pre-washed, dried, and weighed Whatman GFC filter. After adequate rinsing with distilled water to ensure removal of dissolved salts, the filter is dried (103–105 C) desiccated and reweighed.

Filterable reactive phosphorus (mg/L as P)

The current analytical method for the determination of filterable (0.45 micron pore size) reactive phosphorus is Standard Method No. 4500-P (APHA, 2012). The analysis is performed using a segmented flow analyser. The phosphate reacts with ammonium molybdate and antimony potassium tartrate, in acid medium, to form phosphomolybdic acid. This is reduced by ascorbic acid. The absorbency of the resultant blue complex is then measured at 710 nm.

Total phosphorus (mg/L as P)

The current analytical method for the determination of total phosphorus is Standard Method No. 4500-P (APHA, 2012) which follows off-line digestion to convert all forms of phosphorus into orthophosphate using an acidic persulfate digestion. The analysis is performed using a segmented flow analyser. The phosphate reacts with ammonium molybdate and antimony potassium tartrate, in acid medium, to form phosphomolybdic acid. This is reduced by ascorbic acid. The absorbance of the resultant blue complex is then measured at 710 nm.

Oxidised nitrogen (mg/L as N)

The current analytical method for the determination of oxidised nitrogen (NOx) is Standard Method No 4500-NO3 (APHA, 2012). The analysis is performed using a segmented flow analyser.

Nitrate and nitrite (mg/L as N)

The current analytical method for the determination of nitrate and nitrite is Standard Method No 4500-NO3 (APHA, 2012). The conversion of nitrate to nitrite prior to colorimetric determination is achieved using an in-line copper coated cadmium reduction column. The colorimetric reaction involves diazotising the nitrite with sulphanilamide and coupling the N-(1-napthyl)ethylenediamine dihydrochloride. The absorbency is measured at 543 nm.

Ammonia (mg/L as N)

The current analytical method for the determination of ammonia is Standard Method 4500-NH3 H. (APHA, 2012). The analysis is performed using a segmented flow analyser. Ammonia in the sample reacts with alkaline phenol and hypochlorite to form indophenol, an intensely blue-coloured compound, proportional to the ammonia concentration. The blue colour is intensified with sodium nitroferricyanide and the reaction is accelerated by heating. The absorbency of the indophenol is measured at 640 nm.

Total Kjeldahl nitrogen (mg/L as N)

The current analytical method for the determination of total Kjeldahl nitrogen (TKN) is Standard Method No. 4500 NH3 (APHA, 2005). The analysis is performed using a segmented flow autoanalyser. The sample is digested with sulphuric acid and potassium sulphate in the presence of a mercurial sulphate catalyst, followed by an automated phenate colorimetric technique for the determination of ammonium. The ammonium ion produced by the digestion is reacted with salicylate and hypochlorite in a buffered alkaline solution, using nitroferricyanide catalyst. The absorbency of the blue-green complex is measured at 660 nm.
Currently total Kjeldahl nitrogen is calculated by the difference between total nitrogen determined by persulfate digestion within an autoclave followed by the analysis for nitrate and oxidized nitrogen as described above.

Total heavy metals by ICP-MS (mg/L)

The current analytical method for the determination of heavy metals listed below is USEPA Method 6020 and USEPA Method 200.8. Metals are determined by digesting the sample with nitric acid or a combination of nitric and hydrochloric acid at a specified temperature followed by inductively coupled plasma with mass spectrometric detection (ICPMS). Metals analysed include aluminium, arsenic, cadmium, chromium, copper, iron, lead, manganese, molybdenum, mercury, nickel, selenium, and zinc.

Alkalinity (mg/L as CaCO3)

The current analytical method for the determination of total alkalinity is Standard Method No. 2320B (APHA, 2012). Alkalinity is determined using an acid/base titration using standardised sulphuric acid. The analysis is performed on a autosampler combined with a pH meter and an auto burette.
For samples having a pH of 8.3 or less (which is most samples) all the alkalinity is present as bicarbonate. For samples having a pH above 8.3, the alkalinity contributed by hydroxide, plus half of the carbonate present, is determined by titration with standard acid solution to pH 8.3. The total alkalinity is determined by titration to pH 4.6. The concentrations of hydroxide, carbonate and bicarbonate alkalinities are then calculated from the table of relationships given in Standard Method No 2320B (APHA, 2012).

Chloride (mg/L)

The current analytical method for the determination of chloride is USEPA Method 325.2 using a discrete analyser. Thiocyanate ion is liberated from mercuric thiocyanate through the sequestration of mercury by the chloride ion to form un-ionised mercuric chloride. In the presence of the ferric ion the liberated thiocyanate forms highly coloured ferric thiocyanate at a concentration proportional to the original chloride concentration.

Sulphate (mg/L as SO4)

The current analytical methods for the determination of sulphate is USEPA Method 375.4 using a discrete analyser. Sulphate ions are precipitated in an acidic medium with barium chloride to form barium sulphate crystals of uniform size. Light absorbance of the suspension is measured by a photometer and the sulphate concentration is determined by relation to a standard curve.

Fluoride (mg/L)

The current analytical method for the determination of fluoride is Standard Method No. 4500 F- (APHA, 2012). The analysis is performed using an ion selective electrode which consists of a single-crystal lanthanum fluoride membrane and an internal reference bonded into an epoxy body. The crystal is an ionic conductor in which only fluoride ions are mobile. When the membrane is in contact with a fluoride solution, an electrode potential develops across the membrane, relative to the concentration of free fluoride ions in solution.

Calcium (mg/L)

The current analytical method for the determination of calcium is Standard Method No. 3120B (APHA, 2012). The analysis is performed using an inductively coupled plasma optical emission technique (ICP-OES).

Magnesium (mg/L)

The current analytical method for the determination of magnesium is Standard Method No. 3120B (APHA, 2012). The analysis is performed using an inductively coupled plasma optical emission technique (ICP-OES).

Potassium (mg/L)

The current analytical method for the determination of potassium is Standard Method No. 3120B (APHA, 2012). The analysis is performed using an inductively coupled plasma optical emission technique (ICP-OES).

Sodium (mg/L)

The current analytical method for the determination of sodium is Standard Method No. 3120B (APHA, 2012). The analysis is performed using an inductively coupled plasma optical emission technique (ICP-OES).

Silica (reactive) (mg/L as SiO2)

The current analytical method for the determination of reactive silica is Standard Method No. 4500 SiO2 D-E (APHA, 2012) by an automated colorimetric analysis on chilled (not frozen) samples. The analysis is performed using a segmented flow analyser. Soluble silica species react with molybdate under acidic condition to form a yellow silicomolybdate complex. This complex is subsequently reduced with stannous chloride to form a heteropoly blue complex which is measured spectrophotometrically at 880 nm.

Total carbon and dissolved organic carbon (mg/L as C)

The current analytical method for the determination of total organic carbon (TOC) (non-purgeable organic carbon) is an automated procedure which acidifies the sample then the inorganic carbon is removed by purging with nitrogen. Buffered persulfate is added, irradiated with UV and hydroxylamine is added and the sample enters the dialyser. The generated carbon dioxide diffuses through a gas permeable silicone membrane and a weakly buffered phenolphthalein indicator solution is used as the recipient stream wherein the colour intensity will change as the pH varies by the absorbed carbon dioxide gas. The colour intensity is measured at 550 nm.
Dissolved organic carbon can be determined by filtering the unacidified sample through a 0.45µm pore diameter filter and then treating as for TOC.

Chlorophyll-a and Phaeophytin-a

The current analytical method for the determination of chlorophyll-a is Method ISO/DIS 10260 (International Organisation for Standardisation (ISO), 1991). Planktonic algae holding chlorophyll and associated photosynthetic pigments are removed from water samples using vacuum filtration. The filtered residue is extracted into hot ethanol. The chlorophyll-a and phaeophytin-a concentrations in the extract are then determined spectrophotometrically.

Turbidity (NTU)

The current analytical method for the determination of turbidity is Standard Method No. 2130B (APHA) The method is based upon a comparison of the intensity of light scattered by the sample under defined conditions with the intensity of light scattered by a standard reference suspension. A standard suspension of Formazin is used to calibrate the instrument.

Total dissolved solids (mg/L)

The current analytical method for the determination of total dissolved solids is Standard Method No. 2540C (APHA). A representative portion of the water sample is filtered through a 1 micron pore size (nominal) glass fibre filter into an evaporating dish which is then dried at 180ºC and reweighed with the increase in weight being the total dissolved solids present in the portion of sample analysed.

Total recoverable hydrocarbons (TRH) (mg/L)

The current analytical method for the determination of total recoverable hydrocarbon (TRH) is US EPA SW-846 Methods 8015. Water samples are extracted with dichloromethane (DCM) via a liquid - liquid technique and the extract analysed via GC-FID. DCM/acetone and the various fractions of hydrocarbons are analysed with a GC using a flame ionisation detector (FID). Quantification of TRH >nC10 to nC16, >nC16 to C34and >nC24 to nC40 alkanes are carried out by measuring individual or grouped FID peak areas, against the calibration standards made from individual n-alkanes.

Organochlorine pesticides (OCPs) (mg/L)

The current analytical method for the determination of organochlorine pesticides is US EPA SW846, Methods 8270C, 8081, 3510, 3540 (APHA). Water samples are extracted with dichloromethane (DCM) via a liquid extraction and soils are chemically ‘dried’ with sodium sulphate before being extracted with DCM/acetone in an end-over-end tumbling action. An internal standard is added to the DCM or DCM/acetone extract prior to analysis on a gas chromatograph fitted with a mass selective detector (GC/MS). The responses from the sample extracts are identified and compared with standards of known concentrations. To achieve low limits of reporting, the organic solvent extract is concentrated via a technique (Kuderna–Danish) which removes more than 95 % of the solvent volume without significant losing any of the analytes of interest. This concentrate then undergoes the same determinative steps as a normal extract.

Organophosphate pesticides (OPPs) (mg/L)

The current analytical method for the determination of organophosphate pesticides is US EPA SW 846, Methods 8270C, 8140, 8141, 3510 (APHA). Water samples are extracted with dichloromethane (DCM) via a liquid extraction and soils are chemically ‘dried’ with sodium sulphate before being extracted with DCM/acetone in an end-over-end tumbling action. An internal standard is added to the DCM or DCM/acetone extract prior to analysis on a gas chromatograph fitted with a mass selective detector (GC/MS). The responses from the sample extracts are identified and compared with standards of known concentrations. To achieve low limits of reporting, the organic solvent extract is concentrated via a technique (Kuderna–Danish) which removes more than 95 % of the solvent volume without significant losing any of the analytes of interest. This concentrate then undergoes the same determinative steps as a normal extract.

Bromide (mg/L)

The current analytical method for the determination of bromide is US EPA Method No 300.0B, US EPA 1991). This method uses ion chromatography, with chemically suppressed conductivity detection, to determine the anions chlorite, bromate, bromide and chlorate in clean low salinity (<500µS/cm) water samples.

Biochemical Oxygen Demand (BOD) (mg/L)

The current analytical method for the determination of biochemical oxygen demand (BOD) is Standard Method No 5210B (APHA). Samples are diluted with aerated dilution water saturated with oxygen containing nutrient salts and seeded with bacteria. The sample so prepared, is incubated for five days at 20 degrees Celsius in the dark. The depletion in dissolved oxygen concentration during the incubation period yields a measure of the BOD.