# Efficiency and representativeness of suspended particulate matter time-integrative sampling in rivers using particle traps

Matthieu MASSON, Pascaline DIEUDE, Aymeric DABRIN, Céline BERNI, Yann BONENFANT, Alexandra GRUAT, Loïc RICHARD, Jérôme LE COZ, Marina COQUERY

INRAE, UR RiverLy, 5 rue de la Doua, CS 20244, 69625 Villeurbanne Cedex, France

## 1. Context

* Monitoring hydrophobic organic contaminants and trace metals elements (TMEs) in surface freshwaters requires measuring contaminant concentrations in the particulate fraction of the water column.

* Over the past two decades, particle traps (PTs) have been developed to sample suspended particulate matter (SPM) as a cost-efficient, easy-to-operate and time-integrative tool.

* Since 2009, the Rhône Sediment Observatory (OSR) Program[1] has relied on PTs for the monitoring of particulate contaminants throughout the Rhône River and the outlets of its main tributaries.

* The representativeness of particles collected with PTs is still questionable and not fully understood : PTs sample more coarser particle than those of the rivers[2],[3] but with no effect on particulate PCB or Hg concentrations[3].

## 2. Objective

The aim of this study is to assess the representativeness of SPM characteristics and associated TMEs in samples collected from a large river using the PTs deployed in the OSR monitoring network.

An experiment under controlled conditions was conducted to study the performance of the PT. The comparison of TME concentrations measured in SPM collected by PT and by continuous flow centrifugation (reference sampling technique) was conducted using long-term monitoring data.

## 3. PTs used in the OSR monitoring network

<figure><img src="image_1.png" /></figure>

The PTs are similar to those described in Schulze et al.[4]:

- high quality (type 316 L) stainless steel box

- 3 holes on the front and back faces allowing water circulation

- 2 baffles induce a decrease of the current velocity within the PT

- decantation of SPM into two sedimentation tanks.

## 4. Mechanisms and trapping efficiency by experiments under controlled conditions

Ex-situ experiment : direct feeding

<figure><img src="image_2.png" /></figure>

In-situ experiment : deployment in lab flume

<figure><img src="image_3.png" /></figure>

Estimation of the particle diameter that are trapped according to the ratio of fall velocity to overflow velocity (black line) :

$$ \text{Efficiency} = 0.5(\tanh(1.2 \log(\frac{\omega_s}{V}) + 1.1) + 1) $$

<figure><img src="image_4.png" /></figure>

ratio of fall speed to overflow speed

Particle size distribution during experiments in the lab flume :

- the particles collected by the PT are coarser than those in the water flow

- the proposed equation allows to correctly estimate the particle size distribution collected by the PT

Volumetric frequencies

<figure><img src="image_5.png" /></figure>

Particle diameter (µm)

## 5. Representativeness of trace metal elements concentrations in samples collected by particle traps

Concentration of TMEs measured in SPM samples collected in the Rhône River (Jons station) using PTs were compared with those measured in SPM collected during the same time period by continuous flow centrifugation (CFC; reference sampling technique).

TME concentrations in the PT were compared to reference concentrations calculated on the basis of the two bordering CFC samples.

Concentrations of the TEi

<figure><img src="image_6.png" /></figure>

Absolute TME concentration errors = difference between PT and reference concentrations

PT samples located between the two black lines (analytical errors) have a TME concentration similar than the concentrations of the reference :

Cu, Pb, Zn : Under-estimation of concentrations in PT compared to CFC

<figure><img src="image_7.png" /></figure>

Cd : Over-estimation of concentrations in PT compared to CFC

<figure><img src="image_8.png" /></figure>

As, Cr, Co, Ni : similar concentrations in PT and CFC

<figure><img src="image_9.png" /></figure>

Percentage of PT samples with concentrations under or overestimated relative to the reference :

<table><thead><tr><th></th><th>Pb</th><th>Cu</th><th>Cd</th><th>Cr</th><th>Zn</th><th>Ni</th><th>As</th><th>Co</th></tr></thead><tbody><tr><td>Over-estimation by PT</td><td>3%</td><td>3%</td><td>40%</td><td>20%</td><td>1%</td><td>11%</td><td>10%</td><td>1%</td></tr><tr><td>Under-estimation by PT</td><td>79%</td><td>71%</td><td>21%</td><td>26%</td><td>40%</td><td>26%</td><td>19%</td><td>6%</td></tr></tbody></table>

Under-estimation of Cu concentrations in the PT samples, could be explained by the degradation of particulate organic matter (POM) inside the PTs, leading Cu release in the dissolved phase.

This process is predominant during low flows in summer when the autochthonous production of easily degradable POM is maximum.

<figure><img src="image_10.png" /></figure>

An in situ experiments conducted during low flow period in summer allowed for a more detailed study of the quality of particles collected by PT compared to particles collected by CFC at regular intervals during the PT deployment period :

<figure><img src="image_11.png" /></figure>

02/09/2019

07/09/2019

12/09/2019

17/09/2019

22/09/2019

## 6. Conclusions

* ✓ Time-integrative PTs collect coarser particles than those in the river, especially when the flow velocity is high.

* ✓ Sampling of larger particles by the PTs does not seem to impact the concentrations of most TMEs.

* ✓ Cu and Zn concentrations are underestimated by the PTs during low flow periods : degradation of organic matter inside the PT may lead to Cu and Zn release in the dissolved phase.

* ✓ Cu and Zn underestimation by PTs in low flow periods, when SPM concentrations are low, should not impact annual TME flux estimation in rivers.

* ✓ PT sampling is a valuable technique to assess reliable spatial and temporal trends of particulate contaminants.

<footnote>[1] http://www.graie.org/osr; [2] Pohlert et al. (2012) *J. Environ. Monit.* 13:1579-1588; [3] Masson et al. (2018) *Sci. Total Environ.* 637-638:538-549; [4] Schulze et al. (2007) *J Soils Sediments* 7:361-367.</footnote>