How to make a WCF REST method entirely asynchronous with the Task Parallel Library?

I am trying to make a WCF REST method entirely asynchronous (I don't want to block anywhere). Essentially I have a simple service with 3 layers: Service, Business Logic and Data Access Layer. The Data Access Layer is accessing a database and it can take several second to get a response back from that method.

I don't understand very well how to chaining of all those method work. Can someone please help me to complete the sample I am trying to write below? I don't understand well the pattern used by WCF and I didn't find much documentation on the subject.

Can someone help me to complete the following example? In addition, how can I measure that the service will be able to handle more load than a typical synchronous implementation?

using System;
using System.Collections.Generic;
using System.Runtime.Remoting.Messaging;
using System.ServiceModel;
using System.ServiceModel.Activation;
using System.ServiceModel.Web;
using System.Threading.Tasks;

namespace WcfRestService1
{
    [ServiceContract]
    [AspNetCompatibilityRequirements(RequirementsMode = 
        AspNetCompatibilityRequirementsMode.Allowed)]
    [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
    public class Service1
    {
        private BusinessLogic bll = new BusinessLogic();

        // Synchronous version
        [WebGet(UriTemplate = "/sync")]
        public string GetSamples()
        {
            return bll.ComputeData();
        }

        // Asynchronous version - Begin
        [WebGet(UriTemplate = "/async")]
        [OperationContract(AsyncPattern = true)]
        public IAsyncResult BeginGetSampleAsync(AsyncCallback callback, 
            object state)
        {
            Task<string> t = bll.ComputeDataAsync();

            // What am I suppose to return here
            // return t.AsyncState; ???
        }

        // Asynchronous version - End
        public List<SampleItem> EndGetSampleAsync(IAsyncResult result)