T urb id ity

T urb id ity

B a ckg ro und

Sedim ent particles are characterized by their size. Sedim ent is the product of erosional and fluvial

T hey range from the finest clays and silt particles to processes. E rosion involves the processes of sand, pebbles, gravels, and boulders. O nce

d eta ch in g sedim ent particles, transporting them sedim ent particles have been introduced to a stream from the original site and eventually depositing system , the sm aller particles (silts and clays) are those particles. Site characteristics such as typically transported as suspended sedim ent in the geology, soils, slope steepness and length, w ater colum n before eventually settling out and

depositing.

Processing and analyzing suspended sedim ent

Sinuosity is the amount that a stream channel

sam ples and data can be com plex and expensive. A

curves or meanders laterally across the land

frequently used substitute for m easuring suspended

surface.

sedim ent is turbidity. Turbidity is relatively easy

a n d in e x p e n siv e to m ea su re a n d is o ften th e b a sis fo r w a ter q u a lity sta n d a rd s a n d c a n b e c o rrela ted w ith su sp en d ed sed im en t o n a site-sp ecific b a sis . M onitoring turbidity can provide valuable inform ation to help understand baseline trends over tim e as w ell as the effects of a specific project on w ater quality. T he nature and attributes of turbidity are described below to aid in data interpretation.

T urbidity varies w ith the num ber and size of particles present in the w ater colum n. T urbidity is defined as the optical property of a sam ple that causes light to be scattered and absorbed.

Since w ater-borne particles other than sedim ent can scatter light (e.g., fine organic m atter, plankton, m icroscopic organism s), turbidity is not a direct m ea su re o f sed im en t in th e w a ter c o lu m n . T he relationship betw een suspended sedim ent and turbidity can vary greatly betw een sites. For exam ple, a w atershed w ith coarse soils m ay have great fluxations in suspended sedim ent, but turbidity m ay rem ain fairly stable. A w atershed w ith fine clay soils m ay have consistently high turbidity, but low concentrations of sedim ent (M acD onald et al. 1991).

T urbidity levels are influenced by the sam e factors as suspended sedim ent w ith the additional com plication of turbidity’s sensitivity to w ater-borne particles other than sedim ent (B row n 1983). In general, turbidity can be expected to increase during high stream flow events, but this w ill vary w ithin a given storm and betw een storm s. For exam ple, the first storm of the year m ay produce higher turbidity levels than a storm of the sam e m agnitude that occurs later in the season. L ikew ise, as stream flow initially rises during a storm event (referred to as the “ rising lim b” of a storm hydrograph), turbidities m ay be high. T he equivalent flow as the stream recedes (the "falling lim b" of a storm hydrograph) m ay produce low er turbidity levels. B ecause of these characteristics, the relationship betw een suspended sedim ent and turbidity m ust be determ ined for each site (B eschta 1980) and a range of flow conditions (B row n 1983).

T he variability in turbidity betw een sites and over tim e can m ake it very difficult to establish a natural or background level. M easurem ent errors can increase this variability as w ell. So it is im portant to use caution w hen draw ing conclusions w ith the m onitoring data about effects of m anagem ent.

T urbidity m easurem ents m ay be m ost useful for project m onitoring. In this case sam ples should be collected upstream and dow nstream of a planned project, before, during and after the project com m ences.

T he m ost com m only used m easurem ent m ethod for turbidity is the nephlom etric turbidity m ethod (Stednick 1991). N ephlom etric m ethods m easure the scatter of light and perform better for high and low turbidities (m easured in N ephlom etric T urbidity U nits or N T U s).

M ento r C o nta cts

A s w ith any m onitoring project, questions w ill com e up that are not answ ered or covered sufficiently in this protocol. T herefore, a group of m entors that are agency experts in m onitoring have been identified. T hese m entors m ay be contacted w ith specific questions about particular m onitoring goals and efforts. Q uestions about turbidity m onitoring should be directed to one of the follow ing:

O D F M onitoring C oordinator L iz D ent (503) 945-7493

E -m ail: L iz.F.D ent@ state.or.us O regon D epartm ent of Forestry 2600 State Street Salem , O regon 97310

DEQ O regon D E Q L aboratory

1712 SW 11th Portland, O R 97201

Statew ide D E Q V olunteer M onitoring C oordinator turbidity m ust be representative of the K aren W illiam s: (503) 229-5983

environm ental conditions being investigated. For

E -m ail: w illiam s.karen@ deq.state.or.us exam ple, if the m onitoring objective is to determ ine the effects of a grazing activity on turbidity, the

N orthw est R egional M onitoring C oordinator sam ple m ust be collected in a location directly L arry C aton (503) 229-5983

affected by the grazing activity (im m ediately

E -m ail: caton.larry@ deq.state.or.us dow nstream of the activity). T he easiest place to obtain the sam ple m ay be a few hundred feet

W estern R egional M onitoring C oordinator dow nstream of the grazing site at a road crossing.

D ennis A des (503) 229-5983

H ow ever, this w ould not provide a representative

E -m ail: ades.dennis@ deq.state.or.us sam ple because the likelihood of capturing other turbidity-generating activities (a dirt road,

E astern R egional M onitoring C oordinator developm ent site, etc) increases and the sam ple is L arry M arxer (503) 229-5983

no longer representative of the grazing activity.

E -m ail: m arxer.larry@ deq.state.or.us